Chapter 5: Practical experiments into ceramic powder binder printing and cementation glazing
Introduction The experimental work presented in this chapter explores the cementation glazing technique in combination with powder binder 3D printing. This work was undertaken as an independent study by the PhD student and constitutes the primary practical contribution of this thesis. The enquiry grew out of the objectives outlined by AHRC funded project (to which this PhD is affiliated to) which is described in chapter one of this thesis.
Out of the three glazing techniques used to produce faience, much less is known about cementation glazing, especially in terms of the chemical reactions that take place during the firing. This alone makes cementation glazing a very interesting technique to explore. In addition to this, there are several characteristics of the material and glazing method that theoretically make this technique very compatible with powder binder 3D printing. These characteristics will be discussed in more detail later on in this section.
The aim of this chapter is to present key practical experiments that were undertaken in the development of a 3D printing cementation process and to demonstrate the capabilities and limitations of the process developed through this investigation. In order to understand this practical work, it is first necessary to provide a more detailed description of the cementation process, specifically the glazing mechanisms at work. Additionally, it is important to draw attention to the rationale behind combining powder binder 3D printing with the cementation glazing technique as this explains the potential benefits of the union of the material, fabrication and glazing process.
This chapter will be split into three sections;
Section one will describe the rationale for combining the cementation glazing technique with the powder binder 3D printing process and will also provide this necessary technical information for understanding the cementation glazing technique.
Section two will present 5 groups of trials, each focused on a different stage of material and process development. This section will conclude with one final body trial that builds upon the successful aspects of the practical investigation and uses a new approach to produce 3D printed ceramics with a glossy surface finish.
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Section one: Cementation glazing and its suitability for powder binder 3D printing
Rationale for the development of a cementation 3D printing process The experimental work presented in this chapter investigates the potential for using 3D powder binder printing to fabricate ceramic objects that can be glazed using the cementation method.
The powder binder printing process utilises a liquid jetted binder and a powdered build material that also acts as an inherent support for objects as they are fabricated. This process is capable of producing artefacts with a higher resolution and of greater geometric complexity than the paste extrusion process described in the previous chapter. A major advantage of using this comparatively dry technique over conventional1 and novel2 paste forming methods is that the poor formability characteristics associated with faience paste are not an issue for the powder binder process. In fact, it has been found in previous work that the powder binder printing process favours ceramic compositions that contain few plastic components.
The use of this wholly new approach to fabricate objects in faience could potentially see the production of unprecedented forms as the limitations imposed by paste forming techniques are avoided.
The cementation method is a self-glazing technique, meaning that the glaze is not applied directly to the surface of the object but instead occurs as an inherent part of the firing process. This may be considered an advantage over both conventional and current ceramic 3D printing techniques as the glaze application does not require a person skilled in conventional glazing methods, which potentially makes this process more accessible to a wider range of users, including those without conventional ceramic glazing skills. The cementation technique is also potentially a single-firing technique which has advantages over current 3D printing process (which may require 3 or more firings to produce a glazed piece) and conventional ceramics (which typically require 2+ firings) in terms of time and cost reductions and energy.
Another potential opportunity relates to the high silica body required for cementation glazing and the resultant poor working properties of the paste it produces. Cementation
1 Such as hand modelling and mould making techniques. 2 Such as the 3D paste extrusion process developed in the previous section.
107 bodies are even more difficult to work with in their paste form than typical efflorescence bodies. Consequently, examples of ancient artefacts produced using cementation glazing are rare and contemporary artefacts are limited to very small simple or crude shapes such as spherical beads. By 3D printing the body material (using the powder binder technique) there is great potential to extend range of cementation glazed objects to include more detailed and sophisticated forms.
Another potential advantage of this process relates to the glaze powder and how it surrounds the object during the firing. Not only does it glaze the object all over, but it also acts as a support during the firing. A previous research project (carried out by researchers at CFPR) concluded that in some cases, the ceramic 3D printing process benefited from the use of supports during the firing which prevented the ceramic objects from slumping and deforming as they reached maturing temperature. These supports (or setters) were fabricated alongside the artefact they intended to support and were bisque fired and in some cases slipped and fired a second time. The potential advantage of the way in which objects are fired in the cementation process is that no additional support structures would be required, as the powder glazes and supports the object in one, making this process potentially more efficient as a specially-designed setter or support would no-longer be required.
As described in Chapter 2, a characteristic of fired cementation glazed objects is a core structure that ranges from being very soft and friable, to homogenous and glassy throughout (depending on the composition and firing conditions). This homogenous and glassy core structure is an appealing property of cementation glazing especially when considered against the porosity issues that are often encountered with 3D printed ceramics produced using the powder binder process. This property could potentially improve the strength of 3D printed ceramic objects by transforming the once porous core into a dense, homogenous structure.
The potential advantages that the cementation 3D printing process could potentially have over the efflorescence 3D extrusion process developed in the previous chapter, the ceramic 3D printing process used by CFPR and conventional clay-based ceramic processes are summarised in table 5.
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Table 5: Glazing techniques and potential advantages Simultaneously Self-glazing, no Low glaze Potential to be Support glazes up to need for firing single fired provided by 1000 small manual glaze temperature and glazed the glaze objects in a application (around powder during single 1000°C) the firing container
Cementation ✓ ✓ ✓ ✓ ✓ glazing
Efflorescence ✓ ✓ ✓ glazing
3D printed ✓ ceramic glazing
Conventional ✓ ✓ ceramic glazing
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Introduction to the cementation glazing technique The cementation technique was used in ancient Egypt and the Middle East to produce faience, most typically for the production of small, simple objects like beads and simple amulets. It requires the use of a high silica body material which once dry is packed in a glazing powder composed mainly of soda, lime and silica and fired in a saggar (a ceramic container) to around 1000°C. Once fired, the friable glaze powder can be crumbled away to reveal the glazed object contained inside. It is possible to simultaneously glaze up to 1000 small objects in a single saggar with this process and the glaze powder can be re-used by mixing half the amount of the used glazing ingredients to the new mix [Tajeddin, 2014].
The glazing mechanisms Until recently, researchers believed that the mechanism at work for the cementation process involved the vaporisation and diffusion of alkalis (contained in the glazing powder) to the siliceous body (of the object) to form a soda-lime-silica glass. It was thought that this glass wets the body and draws away by surface tension from the glazing powder [Wulff, Wulff, and Koch, 1968] and [Vandiver and Kingery, 1987]. A recent experimental study by Matin and Matin (2012) has convincingly demonstrated that there are in fact two glazing mechanisms at work for this technique, which they named the Interface Glazing Mechanism (IGM) and the Chloride Glazing Mechanism (CGM). As its name suggests, the Interface Glazing Mechanism occurs exclusively at interface between the glaze powder and the object surface. During the firing, the silica (on the surface of the object) reacts with the alkali and lime (in the glaze powder) to form a soda-lime-silica glass which coats the surface of the object. As firing continues, the reaction goes deeper into the body of the object. The Chloride Glazing Mechanism (CGM) involves the vaporisation of alkali chlorides and copper during the firing resulting in the characteristic blue glossy surface. The combination of the IGM and the CGM is necessary for successful glazing through cementation.
Capsule Formation A by-product of the IGM is the formation of a capsule that surrounds the object. The capsule is an integral part of the cementation process as it prevents the glaze powder from adhering to the molten glaze that forms on the object surface during the firing. The experimental study by Matin and Matin [2012] suggested that the shrinkage of a faience bead is caused by the migration of silica from the body (of the object) to the surrounding glaze powder. The increased silica in the surrounding glaze powder alters the silica: lime
110 ratio3, which causes glaze powder around the object to partially fuse together. The silica: lime ratio remains unchanged in the rest of the glaze powder and so, once fired the soft and friable material can be easily crumbled away to reveal the capsule and the glazed object contained within it (see figure 51)
Figure 52 illustrates the migration of silica from the object and subsequent capsule formation. Initially silica is lost from all directions to the surrounding glaze powder. As the object contracts, its surface withdraws away from the glaze powder. The underside of the object remains in contact with the glaze powder resulting in continued silica migrating through this face of the object. It has been observed that an extended firing can lead to the disappearance of the object due siliceous body dissolving completely, leading to an empty capsule.
3 The lime (calcium oxide) component of the glaze powder has a high melting point (CaO melting at around 2570°C). In the presence of silica however, calcium can act as a flux depending on its proportion in the mixture. The purpose of a ceramic flux is to lower the melting point at which the body composition begins to fuse together.
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Figure 51: Cementation glazed object (blue) surrounded by capsule that forms as part of the process. [Image credit: M.Matin and M.Matin 2011, with permission from Matin, M]
Figure 52: Illustration of silica migration and capsule formation in cementation process [Image credit: M.Matin and M.Matin 2011, with permission from Matin, M]
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Summary This section has described the rationale for combining the cementation glazing technique with powder binder 3D printing and has pointed to the potential advantages of using this approach has over current ceramic 3D printing processes and conventional ceramic forming techniques. An introduction to the cementation method and its glazing mechanisms has also been presented to provide the essential technical information necessary to undertake the practical work presented next section.
The potential benefits of cementation may be summarised as follows:
1/ the potential to produce glazed ceramic in single firing compared to 2 or 3 by other processes
2/ the potential for glaze powder to support the object during firing
3/ the potential to create homogenous glassy core
4/ the potential for design freedom/unprecedented forms in cementation glazed faience.
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Section two: Cementation 3D printing trials The practical work presented in this section has been subdivided into 5 groups of trials, each focusing on a different aspect of the material and process development.
Trails in group 1 focus on the development of initial glaze and body compositions which were carried using pastes and simple hand forming methods (i.e. not using 3D printing). This work was necessary to provide a starting point and to establish basic practical knowledge upon which to build in the 3D printing trials presented in groups 2 -5. The first 3D printing trials are presented in group 2, using small simple objects to test the feasibility of this approach and to improve the glaze powder composition. Trials that explore the use of different colourants are presented in group 3 and firing tests and object development trials are presented in group 4. Tests that aim to improve the cementation body strength are described in group 5. This section concludes with one final trial, using a non-faience composition that is also single fired and has an attractive semi-gloss surface appearance.
Details of the material preparation, equipment used and firing parameters are outlined at the start of this section, followed by the criteria used to assess the results of each trial. Details of the 3D printing cementation process is outlined after trials in group 1 have been presented, as this first group of trials were undertaken without the use of 3D printing technologies.
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Equipment and material preparation A 310 Plus, powder binder printer supplied by Z Corporation (now owned by 3D Systems) was used to fabricate objects in the cementation body for this process. The cementation body was prepared by combining the ingredients and mixing them in a paint shaker for 12 minutes with metal mixing stars (see figure 53). The mixed powder was then sieved through a 149µm mesh sieve to remove coarse particles. The cementation glaze powder was prepared in the same way except for the final sieving. It was thought that any coarse particles may potentially be of some use in the cementation process, the reason for which is as follows. During the firing, it is believed that molten beads of glaze move through the glaze powder as they are drawn towards the surface of the object. It was hypothesised that this movement could be assisted by the use of some coarse particles in the glaze powder as these would provide a more open structure and have a similar effect to the use of grog in a conventional ceramic body.
Figure 53: Left, paint can and metal mixing stars. Right, paint can loaded into paint shaker for vigorous mix. [Photo by K. Nash]
Once fabricated, objects were placed in a convection oven and dried at 50°C, 50RPM fan speed. Two firing schedules were used for the cementation glazing work presented in this chapter. These are named the ‘regular firing schedule’ and the ‘slow firing schedule’, the details of which can be seen in table 6 and 7.
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Table 6: Regular firing schedule Temperature Decimal (°C) hours 100 1 200 2 300 3 400 4 500 5 600 6 700 7 800 8 900 9 1010 10.2 1010 10.33
Table 7: Slow firing schedule Temperature Decimal (°C) hours 50 1 100 2 150 3 200 4 250 5 300 6 350 7 400 8 450 9 500 10 550 11 600 12 650 13 700 14 750 15 800 16 850 17 900 18 950 19 1010 20.2 1010 20.67
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Figure 54: Firing curves for the regular and slow firing schedules [by K. Nash]
Firing curves for regular and slow firing schedules
1200
1000
Slow firing C)
° 800 curve
600
400 Temperature( Regular firing 200 curve
0 0 5 10 15 20 25 Time (Hours)
The criteria used to assess the success a particular body formulation at each stage of the cementation 3D printing process is outline in table 8.
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Table 8: Criteria used for assessing the body and glaze development at each stage of the cementation 3D printing process
Fabrication Setting De-powdering Saggar preparation Firing Objects removal
The body powder Objects should be strong The body strength must be The objects must have After being fired to around Minimal adhesion should composition must not cause enough to enable handling sufficient enough to enable sufficient strength to be able 1000°C, the saggar should occur between the glaze the layers to shift during as they are removed from objects to be de-powdered to withstand being placed be cooled thoroughly (to powder and the objects.
object fabrication (due to the 3D printer and placed in using hand tools (i.e. into the saggar and room temperature) to The body strength should be the inclusion of plastic an oven (for further drying) brushes) and pressurised air surrounded by glaze ensure the safe removal of sufficiently strong to materials) and enough without incurring any without incurring breakages. powder. In order for glazing objects contained within. withstand initial removal binder should be included to breakages. to occur all over, the from the glaze powder, the provide objects with powder must reach all removal of glaze powder sufficient green strength for external and internal faces, adhesion and other minor the next 5 stages. which requires thorough forces incurred by handling. packing into the glaze Objects should be coated in powder. an even, shiny and smooth glaze.
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Group 1
Trial 1: Initial glaze and body composition test One of the most significant studies to provide insight to the cementation glazing technique was conducted in 1968 by H.E Wulff, who gained access to a faience workshop in Qom (Iran) where glazing using the cementation method was still in use. The composition of the glaze powder used in Qom was published in ‘Egyptian Faience – A Possible Survival in Iran’. The ingredients of which are as follows;
Qom glaze powder recipe
Plant ash: 3 parts Hydrated lime: 3 parts Quartz powder: 2 parts Charcoal: 0.5 parts Copper oxide: 0.5 parts
It is believed that charcoal was added as an impurity and only has a detrimental influence on this process when included in the composition [Matin and Matin, 2012] and so for this reason it has not been included in the present study. The GP1 recipe detailed below provides the materials and percentages used in this replication study. Materials with equivalent properties to the ones stated by Wulff were selected for use in this trial.
Glaze Powder 1 recipe
Silica: 23.5% Calcium carbonate: 35.3% Sodium carbonate: 35.3% Copper carbonate: 5.9% (on op)
The three body compositions used in this trial are shown in table 9. These bodies were formulated based on similar compositions detailed in the literature [Matin and Matin, 2012] [Wulff and Wulff, 1968]. Fused silica (calcined flint) was selected for exploration in the present study as it is compositionally similar to flint, but due to the fact that it is non- crystalline it is safer to handle in powder form. Maltodextrin is an organic binder composed of cellulous and maltodextrin. It is an essential part of the powder binder process and therefore its presence was required in the body composition.
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The body material was prepared by first grinding the dry ingredients in a pestle and mortar and then passing the powder through a 30 mesh sieve (595 microns) five times. Water was added to make a paste which was used to hand form small discs in each of the three body recipes. The discs were left to dry in ambient conditions (21°) for 72 hours.
Recipe: Cementation body Table 9: Body recipes BR1 BR2 BR3
Fused silica 100% Flint 100% Fused silica 50%: Flint 50% Maltodextrin – 12.5% (% Maltodextrin – 12.5% (% Maltodextrin – 12.5% (% addition) addition) addition)
Objects made from body recipe 1,2 and 3 were burried in the glazing mixture and fired in a
3D printed cermic saggar (see figure 55) using the regualr firing scheduel (table 6)
Results The first three stages of the cementation 3D printing process (i.e. fabrication, setting, de- powdering) are not applicable to this group of trials as objects were hand-formed from a paste material. Therefore, the results relate to the assessment of objects after the firing stage. Following firing, and after the saggar was cooled to room temperature. Significant force was required to remove all three test pieces from the glaze powder resulting in the breaking of test pieces BR1 and BR2. The glaze powder had adhered to all three test pieces, the most extreme can be seen in the BR1 and BR2 pieces. The BR3 test piece adhered to the glaze mixture least out of the three samples. The body strength was not strong enough to withstand removal from the glaze powder. Patches of glaze and colour have been formed on the surface of all three objects.
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Figure 55: 3D printed saggar with test discs made from BR1-3 (test piece size approx. 15 x 15 x 5mm) [Photo by K. Nash]
Figure 56: Fired test discs fused to surrounding glaze powder. Left to right; BR1, BR2, BR3 [Photo by K. Nash]
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The partial glazing the all three test pieces using this glaze powder recipe demonstrates that the cementation glazing technique is working here in part. The replication study conducted by Matin and Matin [2012] describes two glazing mechanisms that work in combination with one another in the cementation method. These mechanisms have been described section 1 of this chapter. This trial demonstrates the patchy glaze and colour that one might expect as a result of the interface glazing mechanism (IGM). The IGM involves the diffusion and migration of alkali salts from the glazing powder to the siliceous faience body and the subsequent interactions that take place to form a glaze on the surface. As its name suggests, this mechanism operates exclusively at the interface between glaze powder and body surface. Characteristics of the IGM include a thin glaze coating that is typically whitish blue, with pale blue/ blue patches [Matin and Matin 2012]. The test pieces produced in this trial are indeed coated with a thin white glaze with blue patches.
The chloride glazing mechanism (CGM) is the second mechanism required for successful cementation glazing. The CGM requires the presence of alkali chlorides in the glazing mixture and involves the deposition of glaze and colour on the object by vaporisation of copper and alkali chlorides. The recipe used in this trial did not contain any alkali chlorides and so this may explain why both the colour and glaze was patchy on all three test pieces. The formulation of the glaze powder used in this trial was based on the work conducted by Wulff, which does not explicitly mention an alkali chloride component in the recipe. What is mentioned is the use of plant ash, which is composed of a mixture of alkali salts, including sodium carbonate and more importantly, sodium chloride. Wulff notes that it is the vaporisation of the sodium chloride present in the plant ash that reacts with copper and results in the formation colour on the beads surface. The replacement of plant ash with sodium carbonate in this trial is likely to be the reason why this mechanism failed to occur.
From this trial it can be concluded that all three test pieces have been partially glazed and coloured by the IGM. The lack of an alkali chloride component in the glaze composition is possibly the reason why objects were not glazed all over, as the CGM could not initiate. The next trial tests this theory and adds 4% sodium chloride to glaze powder. It has been stated that the inclusion of 4% sodium chloride is sufficient to initiate the CGM [Matin and Matin, 2012] and for that reason this value was chosen.
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Trial 2: Sodium chloride trial In this trial, 4% sodium chloride was added to the glaze powder to observe the effect that this had on the cementation glazing process.
Glaze powder recipe 2
Silica: 23.5% Calcium carbonate: 35.3% Sodium carbonate: 35.3% Copper carbonate: 5.9% (on top) Sodium chloride: 4% (on top)
Results The glaze powder was sufficiently friable to enable all three test pieces to be removed from the glaze mixture with moderate force. Slight powder adhesion to the underside of all the test pieces was observed. Body strength was sufficient to enable the removal of test pieces from the glaze mixture. A breakage was incurred in one of the test pieces (fused silica body) when an attempt was made to remove adhered powder from the underside of the object using an abrasive ceramic block. All three test pieces have been successfully coated in a shiny turquoise glaze.
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Figure 57: Fired test pieces cementation glazed in GP2 (left) and capsule (right) (size approx. 15 x 15 x 5mm). [Photo by K. Nash]
Figure 58: Underside of flint: fused silica test piece after firing. [Photo by K. Nash]
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This trial demonstrates the important role that alkali chlorides play in the cementation process. A drastic improvement in glaze formation, glaze smoothness and the strength of colour imparted has been observed from trial 1.
The glaze thickness and colour intensity was slightly variable across the surface of all three test pieces. All three test pieces in this trial have a rough and patchy glaze surface on the underside of the object where they have rested during the firing (see figure 58). It has been noted by Tite, Shortland and Vandiver (2008) that for small objects, cementation glazing characteristically produces objects that are absent of any firing marks. In the case of slightly larger objects, rough patches may be present as a result of pressure on the glazing mixture. The undersides of all the test pieces from this trial were rough and patchy as a result of contact with the glaze mixture.
In terms of body strength, all three test pieces were successfully removed from the glaze powder, however this did require significant effort using the metal picking tool to loosen the object from the surrounding glaze powder. It was thought that a more friable glaze mass could be achieved by using calcium hydroxide instead of calcium carbonate in the glaze powder recipe. This assumption was based on results from a relevant replication study by Matin and Matin [2012] which used a different glaze powder recipe for cementation glazing that incorporated calcium hydroxide with good results.
The next trial explores the use calcium hydroxide in the glaze mixture to assess the difference its use has on the friability of the glaze powder compared to calcium carbonate.
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Trial 3: Substitution of calcium carbonate for calcium hydroxide This trial compares the effect of using calcium hydroxide in the glaze powder to improve friability compared to the use of calcium carbonate.
Glaze powder recipe 2 was used in this trial, however the calcium component was either provided by calcium hydroxide or calcium carbonate. Objects were fired using the regular firing schedule.
Results Figure 59 shows the result of test pieces fired in a glaze powder containing calcium hydroxide. The glaze powder was more friable than previous trials which enabled objects to be removed from the glaze powder more easily. Using minimal force at this stage is beneficial as too much force can result in damage being caused to the object. Minor amounts of glaze powder adhesion were observed on these objects, with the majority occurring on the undersides where contact between the object and the glaze powder (due to gravity) continues through the firing. Figure 60 shows objects that were fired in a glaze powder containing calcium carbonate. The objects were not removed from the glaze powder as easily as they were from the calcium hydroxide powder as the powder was denser than the latter. A visual comparison of the test piece from each glaze powder revealed that there was little difference in terms of glaze formation and colour distribution, both of which were good. This trial has shown that the use of calcium hydroxide in this process is preferential to the use of calcium carbonate in the compositions developed so far due to improvements in finish and ease of removal of the object from glaze powder.
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Figure 59: Hand formed objects fired in glaze powder containing calcium hydroxide (size approx. 15 x 15 x5mm) [Photo by K. Nash]
Figure 60: Hand formed objects glazed in glaze powder containing calcium carbonate (size approx. 15 x 15 x5mm) [Photo by K. Nash]
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It is suspected that the reason for achieving better results in terms of ease of object removal using the calcium hydroxide glaze powder is due to the lower decomposition rate of calcium hydroxide compared to that of calcium carbonate and how this is likely to lead to this component being more active at the temperatures created in the firing stage. Due to the success of this trial in reducing glaze powder adhesion to the object, the calcium component of the glaze recipe was provided by calcium hydroxide in all subsequent trails.
Now that a good working body and glaze composition had been established, it was possible to commence the first 3D printing trials. At this stage in the research, the effect of 3D printing objects (using the powder binder process) on the success of the cementation technique was unknown, as this would be the first time that anyone had attempted to combine these two processes. The next group of trials represent the first attempt to 3D objects and glaze them using the cementation technique.
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Group 2
Process outline The cementation 3D printing process is made up of six stages; object fabrication → setting → de-powdering→ saggar preparation → firing → object removal
The object fabrication stage involves the production of an object via the powder binder printing process. The success of this stage is determined by the suitability of the body for powder binder 3D printing (i.e. its low plasticity), the amount of powdered binder included in the body (to provide sufficient green strength) and the optimum performance of the 3D printer.
The setting stage refers to the amount of time that an object is kept warm, to enable the binder to set. Object were first left in-situ in the 3D printer for up to 12 hours and then placed in a temperature controlled drying oven and heated to 70 degrees (50 RPM fan speed) for up to 24 hours for large objects.
During the de-powdering stage, unbound powder is removed from the surface of the object. It is important that an object has good green strength at this stage so that it can be handled. De-powdering tools include paint brushes of varying hardness and compressed air.
The saggar preparation stage involves the placement of an object into a saggar and then carefully packing the glaze powder around the object, ensuring that all accessible internal and external surfaces are surrounded.
During the firing stage, the saggar containing the objects and the glaze powder is fired to 1000°C.
Object removal occurs once objects have been fired and cooled. Using a pointed metal dental tool, the friable glaze mass is loosened from around the object. Once free, the object is carefully lifted out of the saggar and any powder that remains adhered to the surface of the object is removed using an abrasive ceramic block.
Figure 61 shows photographs that illustrate each stage of this process.
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Figure 61: Photographs of stages 1 – 6 of the Cementation 3D printing process [Photos by K. Nash]
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Trial 4: 3D printing cementation body and glaze trial A small, round bead (approx. 25 x 25 x 25mm) was chosen as a suitable shape for the initial 3D printing trials (designed using CAD software Rhino version 5). The bead was inspired by the Iranian donkey bead, a good luck charm (commonly used to adorn the neck of a donkey) that is still made today in the holy city of Qom (Iran) through cementation glazing.
Objects for this trial were fired using the regular firing program schedule.
3DP body recipe 1
Fused silica: 90% Potash feldspar: 10% Maltodextrin: 12.5% (addition) 4
Glaze powder recipe 3
Flint: 23.5% Calcium hydroxide: 35.3% Sodium carbonate: 35.3% Copper carbonate: 5.9% Sodium chloride: 4% (on top)
Results Beads were successfully fabricated which indicated that the cementation body was suitable for the powder binder printing process. This was to some extent expected as a similar ratio of body ingredients and binder has been used in previous CFPR research – however any change in body recipe or other parameters can have an adverse effect on printing so a successful outcome it is not always guaranteed. Sufficient green strength was provided by the amount of binder used in this composition, enabling objects to be removed from the printer and de-powdered within 3 hours with no breakages incurred. Once fired using the regular firing schedule to 1010°C, beads were removed from the glaze powder using a metal picking tool. Once removed from the glaze powder, most beads showed signs of glaze powder adhesion (where the glaze powder has fused to the surface of the bead). In the majority of cases this was removed or minimised using an abrasive ceramic block, however signs of it remained in the presence of rough patches. In most cases the body
4 The binder is composed of organic matter (sugar and starches) which combusts during the firing. In the final composition, nothing of it remains, which is why is not added as a percentage addition. 131
proved strong enough to withstand removal from the glaze powder and the removal of most of the glaze powder stuck to the surface. In general, the glaze produced was shiny and smooth (except in place where glaze powder had adhered) and the colour imparted was a light turquoise that was evenly distributed. One bead was fired in a glaze powder that used cobalt oxide in place of copper carbonate. This glaze powder produced an interesting but un-even colour distribution on the surface of the bead. This result can be seen in figure 63 along with examples of beads glazed using copper carbonate glaze powder.
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Figure 62: 3D printed bead fired in a ceramic saggar and fired in GPR 3 (size approx. 25 x 25 x 25m) [Photo by K. Nash]
Figure 63: A mixture of 3D printed beads glazed in GPR3 using a mixture of copper carbonate (turquoise beads) and cobalt oxide (blue and white striped bead) colourants. (Large bead size approx. 40 x 25 x 25mm) [Photo by K. Nash]
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Good results were achieved at this initial stage using this body composition. The body was very strong in its green state, enabling easy de-powdering and handling before being fired. Beads were successfully removed from the glaze powder without breaking, however significant glaze powder adhesion was observed on the underside of objects. In some cases, removing the adhered glaze powder resulted in beads being chipped or broken, highlighting that the fired strength of the body has the potential to be improved.
It was thought that the high addition of sodium carbonate in this glaze powder recipe (35%) was causing the glaze powder to be over-fluxed during the firing, which created a more dense mass of glaze powder than was necessary, causing the glaze powder to fuse to the surface of the glaze. It was thought that by reducing the sodium carbonate in the glaze powder these two issues would be improved. A trial was conducted to determine how much sodium carbonate was required in the glaze powder to produce a characteristic faience surface, whilst minimising powder adhesion. This is described in the section that follows.
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Trial 5: Sodium carbonate reduction In order to assess the effect of reducing the sodium carbonate content in the glaze powder, specifically how this might improve the object removal and reduce glaze powder adhesion, six glaze powders were mixed. Objects were 3D printed in the cementation body detailed in the previous trial and cementation glazed using the regular firing schedule. In addition to spherical beads, other shapes were also introduced at this stage (hippo and scarab amulet)
Recipes Table 10: Reduced sodium carbonate recipes a-f GPa GPb GPc GPd GPe GPf
Silica 23.5% 30% 31.2% 32.2% 33.3% 34.3%
Calcium hydroxide 35.3% 44% 45.5% 46.9% 48.3% 49.8%
Sodium carbonate 35.3% 20% 17.5% 15% 12.5% 10%
Copper carbonate (on top) 5.9% 5.9% 5.9% 5.9% 5.9% 5.9%
Sodium chloride (on top) 4.0% 4.0% 4.0% 4.0% 4.0% 4.0%
Results All of the objects produced for this trial were fabricated by 3D printing, set and de- powdered successfully. Once fired, objects were removed from the glaze powder and visually assessed. Several observations were made in this trial relating to the effect of reducing the sodium carbonate in the glaze powder recipe. These were;
1) The glaze powder became more friable as the sodium carbonate addition was reduced, making it easier for objects to be removed. 2) Less powder adhesion was observed on objects as the sodium carbonate addition was reduced. 3) The colour of the objects became darker as the sodium carbonate addition was reduced. A deep turquoise colour is characteristic of cementation glazing and this was achieved on objects fired in glaze powders with 20% – 15% sodium carbonate.
The glaze powder that produced the best overall results in terms of glaze powder friability, minimal powder adhesion to the object and colour was the 15% sodium carbonate recipe
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(see figure 67). The 12.5% and 10% additions produced a less vibrant glaze and a duller turquoise colour than the other recipes.
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Figure 64: 3D printed bead fired in glaze powder containing 35% sodium carbonate addition (size approx. 25 x 25 x 25mm) [Photo by K. Nash]
Figure 65: 3D printed bead fired in glaze powder containing 20% sodium carbonate addition (bead size approx. 25 x 25 x 25mm - scarab size approx. 30 x 15 x 7mm ) [Photo by K. Nash]
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Figure 66: 3D printed bead fired in glaze powder containing 17.5% sodium carbonate addition (bead size approx. 25 x 25 x 25mm - scarab size approx. 30 x 15 x 7mm ) [Photo by K. Nash]
Figure 67: 3D printed hippo amulet fired in glaze powder containing 15% sodium carbonate addition (size approx. 50 x 25 x 5mm) [Photo by K. Nash]
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Figure 68: 3D printed bead fired in glaze powder containing 12.5% sodium carbonate addition (size approx. 25 x 25 x 25mm) [Photo by K. Nash]
Figure 69: 3D printed bead fired in glaze powder containing 10% sodium carbonate addition (size approx. 25 x 25 x 25mm) [Photo by K. Nash]
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The reduction of sodium carbonate in the glaze powder improved powder friability, lessened powder adhesion to the glaze and altered its colour. The change in colour was not expected, however the slightly deeper turquoise achieved with less sodium carbonate (between 20-15%) created a glaze that was more characteristic of traditional cementation glazing (based on the characteristics discussed in Chapter 2). In conventional ceramics high alkaline glazes produce vibrant turquoise colours when used with copper colourants therefore it seems logical that colour vibrancy would decrease as the alkaline component is reduced. This was seen in the 12.5% and 10% addition recipes where the turquoise became duller and darker. The optimum addition was found to be around 15%, where powder friability and minimal glaze powder adhesion was at its lowest without compromising the colour of the glaze. As a result of this trial, subsequent glaze compositions contained 15% sodium carbonate. See below for the complete recipe.
Glaze powder recipe 4
Silica: 32.2% Calcium hydroxide: 46.9% Sodium carbonate: 15% Copper carbonate: 5.9% (on top) Sodium chloride: 4% (on top)
Based on current knowledge, copper has been exclusively used as the colourant for cementation glazing. The reason for this is certainly not due to a lack ceramic colourants available, as there are many ceramic stains and metal oxides that are highly effective and accessible. It is more likely due to a lack of practical investigation into this glazing technique (to the best of the researcher’s knowledge), specifically any study that has aimed to expand the range of colours that can be achieved. The next group of trials explore the use of metal oxides and ceramic stains as potential colourants to be used in this process.
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Group 3
Trial 6: Colour trials In order to explore the range of colour achievable using the cementation 3D printing process, the following four colourants were added to the recipe detailed below; chromium oxide, manganese carbonate, cobalt carbonate and iron oxide.
Cementation glaze powder recipe with colourants
Silica: 32.2% Calcium hydroxide: 46.9% Sodium carbonate: 15% Sodium chloride: 4% (addition) Colourant: 5.9% (addition)
Results All of the objects produced for this trial were fabricated by 3D printing, set and de- powdered successfully. Objects were removed from the glaze powder with minor differences in glaze powder friability and adhesion to the surface. The results of this trial are shown in figure 70. The bead fired in the chromium oxide powder produced a bright green-yellow colour and a glaze that was visible but dry and patchy. The manganese carbonate glaze powder gave the donkey bead a light purple colour and also produced a glaze that was dry and patchy. The cobalt oxide powder produced a rich blue colour and a glaze that was mostly glossy and relatively smooth. The iron oxide powder did not impart any colour5 and produced a dry, patchy glaze surface.
5 The slight turquoise colouration on this bead is due to copper contaminants in the saggar transferring on to the bead during the firing. 141
Figure 70: Donkey beads cementation glazed in coloured glaze powders. Left to right, top row to bottom row; Chromium oxide, manganese carbonate, cobalt carbonate, and iron oxide (size approx. 25 x 25 x 25mm) [Photo by K. Nash]
The colourants used in this trail produced mixed results. Three colourants successfully transferred their colour to the beads during the firing. These were; chromium oxide, manganese oxide and cobalt carbonate. Iron oxide did not transfer any colour on to the bead. It is possible that the reason for this is due to this colourant requiring a higher addition than the other colourants in order to impart the same degree of colour. Likewise, it is possible that the purple given by the manganese colourant could have been made stronger by increasing the amount added to the glaze powder. The glaze surface was not as glossy as that achieved using copper as the colourant, however satisfactory results were obtained from the cobalt colourant which produced a surface that was mostly glossy and fairly uniform. The dry patch surface observed on the other beads suggests that these colourants interfere in some way with glaze formation.
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A different approach used to add colour to glazed ceramic objects is to first stain the body material (using a ceramic stain or metal oxide) and then apply a clear glaze on top. In order to test the feasibility of this idea, it was decided to first attempt to apply a clear glaze onto 3D printed objects through cementation glazing. Presently there are no examples (recent or historic) of this having been achieved on objects produced using traditional paste modelling techniques.
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Trial 6: Transparent glaze In order to determine whether it would be possible to apply a clear glaze onto the cementation body, a glaze mixture with no colourant was prepared. The scarab model was 3D printed in the cementation body and buried in the transparent glaze mixture (this recipe detailed below).
Transparent glaze powder recipe Silica: 32.2% Calcium hydroxide: 46.9% Sodium carbonate: 15% Sodium chloride: 4% (addition)
Results The scarab beetle coated in the transparent glaze is shown in figure 71. The body strength was sufficient to withstand removal from the glaze powder and the removal of any powder that had adhered to the surface. The surface was coated in a glossy, evenly distributed glaze.
Figure 71: Cementation glazed 3D printed scarab coated in transparent glaze (size approx. 35 x 15 x 7mm). [Photo by K. Nash]
This trial suggested that objects could be successfully glazed using the cementation process using a glaze mixture with no colourant added. This showed promise for the next stage of development, where colour was to be added to the cementation body which would then be fired in the transparent glaze tested here.
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Trial 7: Coloured cementation body In this trial, two cementation bodies were prepared; one containing a 5% addition of ceramic stain and the other containing a 10% addition. There were two reasons for choosing to use a ceramic stain rather than a metal oxide as colourant in this trial. 1) Ceramic stains are available in a wide variety of colours, which could potentially extend the range of colours for this process significantly. 2) Ceramic stains are more stable than metal oxides, meaning that they are more likely to give predictable and reproducible results.
3DP body recipe + ceramic stain recipe Fused silica: 90% Potash feldspar: 10% Maltodextrin: 12.5% (on top) Gooseberry ceramic stain: 5% (addition) and 10% (addition)
Results Both tiles were fabricated, set and de-powdered successfully. After firing, the tiles were removed from the glaze powder with ease. Patches of glaze powder adhesion were present on both tiles (see figure 72) which could not be removed using the usual tools. More glaze had formed on the tile glazed in the 5% stain composition than on the tile glazed in the 10% stain composition, however the glaze was generally dry, rough and uneven on both tiles. The colour imparted by the ceramic stain was good in terms of its vibrancy and its likeness to the colour it was supposed to emulate (i.e. Gooseberry).
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Figure 72: 5% stained body (left) 10% stained body (right), cementation glazed (size approx. 30 x 30 x 10mm) [Photo by K. Nash]
The addition of a ceramic stain to the cementation body had a detrimental effect on the cementation glazing process. The 5% stain addition produced slightly better glaze results than the 10% addition. This suggests that a possible reason for the lack of success in this trial could be related to a reduction in the amount of silica in the body due to the proprtion of stain added. This could potentially be minimised by addidng a stronger colourtant to the cementation body that is effective in smaller percentages (e.g. a metal oxide such as cobalt oxide).
The potential of using different colourants in the cementation 3D printing process has not been explored any further in this investigation due to time limitations. Instead development work has focused on establishing basic parameters for 3D printing and improving glaze and body composition to explore the potential forms that can be achieved using these materials and processes.
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Group 4
Trial 6: Gradient kiln test In order to identify the firing temperature that produced the best glaze and surface appearance, objects were fired to different peak temperatures using a gradient kiln with 9 separate chambers and assessed. Donkey beads were first 3D printed and then packed in the glaze powder and fired. The first batch of beads was fired between 920°C – 1000 °C at 10° increments (see figure 73).
The second batch was fired between 1005°C-1045°C at 5°C increments (see figure 74). The beads were then measured to provide an indication of the fired contraction at the temperatures used in this trial. The regular firing schedule was used to reach each peak temperature.
3DP body recipe 1
Fused-silica: 90% Potash feldspar: 10% Maltodextrin: 12.5%
Glaze powder recipe 4
Flint: 32.2% Calcium hydroxide: 46.9% Sodium carbonate: 15% Copper carbonate: 5.9% (addition) Sodium chloride: 4% (addition)
Results All beads from batch 1 were removed from the glaze powder with ease. Very little glaze was formed on the surface of the beads until 970°C was reached. As the peak temperature was raised, the amount of glaze formed on the surface increased and the surface texture became smoother. Beads fired in batch 2 became progressively more difficult to remove from the glaze powder, as the mixture became less friable as the peak temperature rose. The glaze surface of the beads from batch 2 became progressively rougher as peak temperature was increased and some distortion was observed on the bead fired at 1045°C. Additionally the glaze powder adhesion became more difficult to remove from the surface beyond 1025°C. The best results in terms of glaze powder friability were seen in mixture
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fired between 1000°C -1035°C. Powder adhesion was minimal on objects fired between 930°C - 1030°°C, after which excessive force resulted in some of the beads breaking. The best glaze results in terms of surface smoothness, colour and gloss were seen on beads fired between 1000°C -1020°C.
The percentage shrinkage of the beads fired in batches 1 and 2 are shown in table 11 and 12. When fired between 1000°C -1020°C, the percentage shrinkage is 15-20%. Between 1020°C – 1045°C, only 0.5% additional shrinkage was observed. Figure 75 shows a graph of the results from this trial.
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Figure 73: Batch 1 - Beads fired between 920°C-1000°C fired at 10°C increments (size approx.. 25 x 25 x 25mm) [Photo by K. Nash]
Figure 74: Batch 2 - Beads fired between 1005°C-1045°C fired at 5°C increments (size approx.. 25 x 25 x 25m) [Photo by K. Nash]
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Table 11: Percentage shrinkage of beads fired in batch 1
Batch 1 Temperature (°C) Shrinkage (%) 920 13.0 930 13.0 940 13.0 950 13.0 960 15.0 970 15.0 980 15.0 990 15.0 1000 15.5
Table 12: Percentage shrinkage of beads fired in batch 2
Batch 2
Temperature (°C) Shrinkage (%)
1005 16.5
1010 18.0
1015 18.5
1020 20.0
1025 20.0
1030 20.5
1035 20.5
1040 20.5
1045 20.5
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Figure 75: Percentage shrinkage of cementation glazed objects fired at different peak temperatures
Percentage shrinkage of donkey beads cementation glazed at different peak temperatures 25
20
15
10
5 Percentage shrinkage (%)
0 900 920 940 960 980 1000 1020 1040 1060 Peak temperature (°C)
This trial has established the firing range of 1000°C -1020°C produces a friable glaze mixture, minimal glaze powder adhesion and the best glaze results in terms of surface smoothness, colour and gloss. Beyond this temperature, the surface becomes rough and the glaze mixture loses friability. Below this temperature, very little glaze is formed and the surface is also rough and uneven. The most shrinkage was observed between 1000°C- 1030°C, after which the beads fired between 1020°C -1045°C only shrank by half a percent. This suggests that a significant amount of silica migration from the bead to the surrounding powder (responsible for capsule formation) occurs between 1000°C-1030°C. The fact that the beads shrank very little beyond this temperature could explain why the glaze powder became more difficult to remove as the peak temperature increases. The migration of silica is responsible for the capsule formation around the object. As the silica migrates, the object withdraws away from the surrounding powder. If the temperature continues to increase once the object has stopped shrinking, it might be expected that any contact between the object and the surrounding powder could fuse together, making a stronger bond, that is more difficult to remove once fired.
The peak firing temperature of 1010°C was selected for subsequent trials based on choosing the midpoint from the 1000°C -1020°C range identified as producing the best results in this trial.
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Through this practical investigation, a suitable body and glaze powder composition for the cementation 3D printing process has been established and the optimum firing temperature for best glaze results has been identified for small objects up to approx. 25mm size. The next logical step in the development of this material and process is to attempt to fabricate larger more complex forms in order to discover potential challenges of using this approach and to develop ways to overcome these them.
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Trial 7: Object development The cementation 3D printing process offers the potential to produce larger and more complex forms than have previously been produced using traditional paste modelling techniques. The reasons for this have been discussed at the start of this chapter.
As with 3D paste extrusion, there are several factors effecting size and shape capacities of this material and process. In order to understand these factors further, it is necessary to look at each stage of the cementation 3D printing process in relation to potential size and shape restrictions.
Object fabrication A major factor restricting the size of objects produced using the powder binder process relates to the size of the build platform. The particular 3D printer used in this research has the following dimensions maximum build volume 203 x 254 x 203mm.
Additionally, the minimum feature size is dictated by both the thickness of each build layer (approx. 0.1mm) and the size of the particles that constitute the body composition (approx. 75µm for the composition used here).
Setting Once objects have been fabricated they must remain in-situ for a period of time to allow the binder to set hard. Before the binder has set, objects are very fragile and rely on the surrounding (unbound) powder for support. Depending on shape, objects with drastic curvatures or large overhangs may not be suitable for this process as such objects may not be able to support their own weight until setting is complete. The amount of time required for objects to set is dependent on their size, mass and complexity. Intricate or large chunky parts will require more setting time to give them the best chance of surviving the remaining stages of the process.
De-powdering The green (unfired) strength of the body plays a key role in determining size and shape capabilities of this process. Once set, objects are removed from unbound material and excess powder is removed using brushes and compressed air. At this stage it is important that a body has good green strength to survive the minor forces applied when de- powdering and handling. A composition that possesses good green strength has greater potential for the production of larger, more intricate forms.
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Saggar preparation Cementation glazing requires the use of a saggar which contains the object and glaze powder during the firing. The size of the saggar poses a restriction on the size of the object, therefore the bigger the object, the larger the saggar must be to accommodate it. Additionally, the bigger the saggar, the more glaze material required to fully surround the object, adding time and material costs to the process.
Firing During the firing, a series of chemical reactions result in the formation of a smooth, brightly coloured glaze that evenly coats the objects. A result of these chemical reactions is that the body material shrinks away from the surrounding powder. Depending on object shape, there is a risk that the glaze powder might temporarily restrict the object from shrinking, which could result in the weakening or destruction of certain features. This could potentially limit the intricacy of objects produced using this technique. It is also worth mentioning that the increased mass of material (object, saggar and glaze powder) that accompanies firing larger objects is likely to require an increased amount of energy during the firing stage. This might mean that firing parameters need adjusting to accommodate larger objects to initiate the same reactions during the firing.
Object removal Once fired, the body composition must be strong enough to survive being removed from the glaze powder. Objects that are larger and/or more delicate require a stronger body in order to survive the firing process and removal from the saggar. If this is not the case, changes to the body composition, glaze powder and firing parameters may be required to improve the fired body strength.
This trial explored the production of four different objects;
A Goblet – Size approx. 100 x 80 x 80mm
A Torque Necklace – Size approx. 155 x 155 x 40mm
A Hippo Statuette – Size approx. 70 x 40 x 30mm
A small Lattice Pyramid – Size approx. 20 x 15 x 15mm
These objects were selected based on differences in their sizes, geometries, and complexities and were influenced by typical Egyptian faience artefacts. Figures 76 - 83 show these objects during the final part of the process – object removal. 154
Figure 76: The friable glaze powder is removed from around the goblet [Photo by K. Nash]
Figure 77: The goblet is carefully removed from the saggar (size approx. 100 x 80 x 80mm) [Photo by K. Nash]
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Figure 78: Fired saggar containing large necklace – still surrounded by the glaze powder [Photo by K. Nash]
Figure 79: Necklace removed from saggar with glaze powder adhesion present on one side – note the necklace has been broken during its removal from the glaze powder as can be seen from the crack running across the surface (size approx. 155 x 155 x 25mm) [Photo by K. Nash]
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Figure 80: The brightly glazed hippo is revealed within the saggar [Photo by K. Nash]
Figure 81: Glazed Hippo with broken leg (approx. 70 x 40 x 30mm). Damaged incurred during removal of glaze powder adhesion [Photo by K. Nash]
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Figure 82: Saggar containing fired lattice pyramid [Photo by K. Nash]
Figure 83: 3D printed, cementation glazed lattice pyramid (size approx. 20 x 15 x 15mm). One of the struts have been broken during object removal [Photo by K. Nash]
All objects were built, de-powdered and fired without being damaged or broken. This demonstrates that all these geometries were 1) suitable for the powder binder printing process, 2) capable of supporting their own weight during setting, 3) had sufficient green strength to withstand de-powdering and handling 4) were adequately packed into a suitably sized saggar and completely surrounded by glaze powder and 5) underwent the chemical reactions necessary for cementation glazing.
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Issues relating to body strength were revealed during this final stage of the process. Three out of the four objects were damaged either during object removal from the saggar or as glaze powder adhesion was removed. A large section of the necklace snapped off as glaze powder adhesion was removed from one side. One of the back legs of the hippo snapped as glaze powder was removed from its underside. A strut at the base of the pyramid snapped as the object was lifted out of the glaze powder. The goblet remained in-tact, however glaze powder adhesion was only partially removed in order to keep the object in one piece.
By handling these objects, it became clear why they were so fragile and prone to breaking. An investigation of the underlying body revealed that it was so soft that it could be easily rendered into a powder by rubbing a fingernail backwards and forwards across the surface several times.
The fact that all objects were glazed successfully indicates that compositionally, the glaze and body are compatible with one another and will result in successful cementation glazing. One observation of this trial was that the surface finish of the larger objects (goblet, necklace and hippo) was less smooth and uniform than had been seem in earlier trials with smaller objects. A possible reason for this could be due to their greater mass (especially when considered inside their saggar’s) which is likely to require more energy input during firing to bring about the necessary chemical reactions for cementation glazing. It was thought that for larger objects, a slower firing rate and extended soak time may improve the glaze surface. This slower firing schedule is detailed at the start of this section.
In order to develop these material and process further, the body strength must be improved without drastically altering the composition of the glaze or body. This is to ensure that successful cementation glazing continues.
The next group of trials explore 3 different ways in which this could be achieved.
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Group 5
Trial 8: Alternative/additional flux In order to improve the cementation body strength, an alternative flux was added to the body. Talc was selected for this trial, as in small amounts (1-5%) it can act as a powerful flux in some bodies [Hansen, Talc, 2003] therefore it was thought possible that it may be of use here, as it could potentially have maximum effect with a minimum change to the body composition. In addition to this, the percentage of the flux presently used in the cementation body (potash feldspar) was increased in order to determine whether this might improve the body strength without significantly effecting the cementation process. The soft friable core that is characteristic of cementation bodies is primarily due to the main component (silica) having a melting point (1830°C) that is far higher than the peak temperature used in this process (1010°C). The role of the flux is to reduce the temperature at which the whole body begin to fuse together, therefore its use here could potentially increase the amount of fusion that takes place in the body at the temperatures used in cementation glazing.
A small bead (size approx. 25 x 25 x 25mm) was 3D printed in each of the new body recipes. Each bead was then broken in half to assess and compare the core structure (see figure 84). The regular firing schedule (detailed at the start of this section) was used for this trial.
Recipes BRa BRb BRc Fused silica 98.0% 90.0% 80.0% Talc 2.0% 0.0% 0.0% Potash feldspar 0.0% 10.0% 20.0% Maltodextrin 12.5% 12.5% 12.5%
Results All objects were successfully fabricated, set and de-powdered demonstrating that all three body compositions suited the powder binder printing process and that each body had sufficient green strength to enable removal from the printer and de-powdering. The glaze powder was sufficiently friable to enable all three objects to be removed after firing. The glaze surface and colour was good on BRa and BRb, however BRc produced a comparatively unattractive surface that was bumpy and rough in texture (see figure 85). The glaze powder
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had adhered in places to objects made in all three body recipes. An examination of the core of each body recipe revealed that BRb was slightly harder than BRa and that BRc was significantly harder than both of these two formulations.
Figure 84: Cross sections of objects 3D printed in recipes BR4 -BR6 [Photo by K. Nash]
Figure 85: Objects made using BR6 recipe (Bead size approx. 25 x 25 x 25mm, Scarab size approx. 35 x 15 x 7mm, hippo size approx. 30 x 20 x 15mm) [Photo by K. Nash]
This trail showed that objects with the softest core were produced using the BRa recipe. The BRb recipe produced objects with a slightly harder core than BRa. BRc produced the best result with a core that was significantly harder than both of the other body recipes. However, the surface appearance of BRc was un-attractive as the glaze was not very shiny and the surface was rough and bumpy. The lack of glaze forming on objects made in the
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BRc recipe is possible due to their being less silica in the body which assists in glaze formation.
As a result of this trial it was decided to continue to use the BRb recipe (containing 10% flux) as this composition possessed good green strength and was capable of achieving an attractive glaze and surface appearance.
The peak firing temperature used in the cementation process is around 1000°C. For a body composed mainly of silica, this is a very low temperature to expect much vitrification to have occurred once fired. One solution to improving the body strength would be pre-fire the body material to a much higher temperature and then fire it a second using the cementation process. This idea is explored in the following trial.
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Trial 9a: Bisque firing to improve body strength To investigate a way to potentially improve the body strength of cementation glazed ceramics, 3D printed objects were first bisque fired before being cementation fired. The body composition contains 10% flux (potash feldspar) however it was suspected that this component was having very little fluxing effect on the body at the temperatures reached in the cementation firing (1010°C). It was therefore decided to bisque fire objects to a higher temperature than that achieved in the cementation firing before glazing. It was thought that this would allow the flux to act on the body, making it stronger without altering the composition, which has shown to yield good results in terms of green strength and glazed surface appearances.
The cementation body was first bisque fired to 1190°C prior to being fired for a second time for cementation glazing. The details of the bisque firing schedule can be seen in table13 and figure 86.
The effect of bisque firing on the surface quality of the object was assessed using the hippo model. Two objects were fabricated, one of which was bisque fired and another that was not. Both objects were then cementation glazed and compared.
Table 13: Bisque firing schedule for cementation body Temperature (°C) Time (decimal hours) 100 1 200 2 300 3 400 4 500 5 600 6 700 7 800 8 900 9 1000 10 1100 11 1190 11.9 1190 12.32
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Figure 86: Bisque firing schedule, time against temperature
Bisque firing time against temperature
1400
1200
C) 1000
° Bisque firing 800 curve 600
Temperature( 400
200
0 0 5 10 15 Time (decimal hours]
Results Both objects were removed from the glaze powder with ease suggesting that bisque firing had no negative impact on the glaze powder friability. The hippo statuette that had not been bisque fired (figure 87) had significant glaze powder adhesion on several surfaces that could not be removed from the surface. The general surface appearance of the non-bisque hippo was uneven, as it was glossy in places but also rough in other places and dull due to glaze powder adhesion. The glaze powder that adhered to the bisque fired hippo was limited to its underside and was removed with remarkable ease (compared to the usual effort required) with an entire section coming away from the surface in one piece (see figure 88). The bisque fired hippo had a good surface finish and was evenly coated in a deep turquoise glaze. In terms of surface quality, the bisque fired hippo appeared more even and smooth than the non-bisque fired hippo, indicating that a greater degree of consistency might possibly be achieved through bisque firing. A section of each hippo was removed in order to examine and compare the core structure. The non-bisque fired hippo was easily marked with a light pressure and the bisque fired hippo required a greater pressure to make an impression on the core, supporting the expectation that bisque firing would improve the fired body strength.
Darker ‘pools’ of glaze were visible on the surface of both hippo statuettes. This glaze ‘pooling’ effect is a characteristic of cementation glazing, which is believed to occur as a
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result of glass transformation acting on the surface. This quality is seen on traditionally made cementation glazed artefacts, examples of which can be found in chapter 2.
Figure 89 shows two 3D printed, cementation glazed scarab beetles. The left hand beetle was single fired and the right hand beetle was pre-fired to 1190°C. The left hand beetle has lost some of its fidelity due to significant glass transformation around its edges. The right hand beetle has retained its shape better by comparison. The pre-fired beetle is slightly lighter in colour compared to the left hand beetle and has fewer, smaller pools visible on its surface.
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Figure 87: Single fired hippo statuette [Photo by K. Nash]
Figure 88: Hippo statuette, bisque fired to 1190°C and then cementation glazed. [Photo by K. Nash]
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Figure 89: Scarab beetles, 3D printed in cementation glazed. Left hand scarab single fired, Right hand scarab, pre-fired to 1190°C. (Size approx. 65 x 35 x 35mm) [Photo by K. Nash]
The effect of bisque firing an object before cementation glazing has been assessed in this trial. Results indicate that bisque firing improves the strength of objects without effecting the glaze formation and actually improves object fidelity, producing better glaze consistency across the surface. These results also indicate that bisque firing reduces glaze powder adhesion to the object surface which is another potential benefit of pre-firing the body composition before cementation glazing. A potential reason for reduced glaze powder adhesion and a better glaze surface might be due to the silica in the body being less active as a result of pre- firing, having the effect of creating more even and controlled reactions across the surface during the firing.
The use of this new approach could potentially enable the production of larger objects with more complex geometries than the objects produced so far in these practical experiments.
A further characteristic of cementation glazing is that under some conditions i.e. when formed in particularly thin sections the body can be transformed into a glass. The effect that the glassy composition has on the visual properties of fired objects is an added degree of translucency, which enhances the optical properties of the material, whilst maintaining the visual characteristics of the cementation glazing process.
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The extent of glass formation is largely dictated by the object thickness, the firing temperature and firing time. In some cases within this research, parts of an object have been known to disappear entirely, as a result of continual silica migration, which ultimately results in the object being absorbed into the capsule forming around it. The extent of glass formation is difficult to control. For example, an object composed of sections that are identical in thickness can result in some sections turning to glass, and other sections disappearing entirely (see figure 90).
It was thought that bisque firing objects before cementation glazing could provide a greater degree of consistency and control in relation to this glass transformation property. The thinking behind this idea relates to the observations made in this trial and the hypothesis that bisque firing may result in more controlled and even reactions to take place during the firing. This idea was explored in the next trial.
Figure 90: 3D printed beads, cementation glazed in GP2 mixture (size approx. 20 x 20 x 20mm) [Photo by K. Nash]
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Trial 9b: Bisque firing to improve glass formation The aim of this trial was to assess whether bisque firing objects before cementation glazing could provide a greater degree of consistency and control in terms of glass formation. A lattice pyramid object (size approx. 50 x 55 x 55mm) composed of 4mm ‘struts’ was chosen for this trial. Two objects were fabricated, set and de-powdered in the usual way. One of the objects was bisque fired to 1190°C and then both objects were cementation fired using the slow firing schedule detailed at the start of this section.
Results Both objects were removed from the glaze powder with ease. The pyramid that had not been bisque fired (figure 91) had significantly more glaze powder adhering to the surface than the bisque fired object. Additionally, sections of the non-bisque pyramid were missing as a result of being absorbed into the glaze powder. It was also observed that this model had visibly distorted during the cementation firing. The pyramid that had been bisque fired prior to the cementation glazing firing was complete, i.e. with no sections missing and had distorted very little in comparison (see figure 92)
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Figure 91: Lattice pyramid, 3D printed in cementation body BR5 and then cementation glazed in GP2 glaze mixture. [Photo by K. Nash]
Figure 92: Lattice pyramid, 3D printed in cementation body BR5, bisque fired to 1190°C and then cementation glazed in GP2 glaze mixture. [Photo by K. Nash]
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This trial has shown that more control can be achieved on the transformation of thin sections into a more glassy composition by bisque firing objects before cementation glazing. This suggests that the effect of pre-firing slows down the rate of glass formation to produce a complete model with a more uniform surface finish. Less distortion had also been observed on the pyramid that was bisque fired which is also of benefit.
In addition to bisque firing the cementation body, it was thought the fired strength could also be improved by infiltrating objects with colloidal silica. Colloidal silica is used in the foundry industry as a high temperature binder, and to add strength and density to refractory materials [Grace, 2008]. It was thought that this would increase the density of the object by adding more silica, potentially improving the object strength whilst maintaining the chemistry (most importantly, the high silica content) of the body. This idea was explored in the next trial.
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Trial 10: Ludox infiltration In an attempt to improve the strength and density of the body the cementation body, objects were infiltrated with colloidal silica, using a commercially supplied solution LUDOX (420840, supplied by Sigma Aldrich). It was thought this would increase the density of the cementation body without changing the composition, specifically the high silica body requirements for cementation glazing. The cementation body had to be pre-fired for this trial. If it had not been the organic binder would dissolve in the LUDOX solution, resulting in the destruction of the object. The hippo model was used for this trial, which was fabricated, set and de-powdered in the usual way and then bisque fired to 1190°C. Objects were then submerged in the LUDOX solution for 1 minute, dried (for 1 hour at 110°C) and then dipped and dried a second time for 1 minute. The hippo was weighed before and after being dipped to monitor this increase in density. These values are displayed in table 14.
Table 14: Object weight before and after infiltration Weight (g) before Weight (g) after 1st Weight (g) after 2nd infiltration infiltration infiltration
19.5g 26.4g (35.4% increase) 29.3g (11% further increase)
The LUDOX solution increased the weight of the object by 35% after the first infiltration and by a further 11% after the second infiltration. This was a total of 50% overall from the weight before infiltration (19.5g). The object was placed in a saggar, buried in the glaze powder and fired.
Results The results of this trial can be seen in figure 93. The object was removed from the glaze powder with relative ease. The use of LUDOX did not seem to affect the friability of the glaze powder. Some glaze had formed on the surface, however this was patchy and dry compared to non-infiltrated examples. Additionally, the colour of the glaze was much lighter than non-infiltrated examples which is likely to relate to the fact that less glaze has formed on the surface. Significant glaze powder adhesion was observed on the underside of the object, which could not be removed using the usual methods.
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Figure 93: LUDOX infiltrated 3D printed hippo statuette, cementation glazed. [Photo by K. Nash]
The LUDOX infiltration has increased the density of the object by 50%. The body strength was notably stronger than non-infiltrated objects processed under the same conditions, suggesting potential for further developing this idea.
In ongoing research more success has been had using this approach, including improved density, strength and glaze finish. A lengthened infiltration time (24 hours) in the LUDOX solution has led to some very promising results (see figures 94 - 95). This development work has taken place immediately after the research described within this thesis and builds upon its findings, using the materials and methods developed through the practical investigation presented in this chapter. In addition to improved body strength, less glaze powder adhesion has been observed on these infiltrated examples compared objects that have been single fired and bisque fired before being cementation glazed. This is also very significant in the overall development of this material and process as it means that the glaze will be smoother, more even and potentially more consistent.
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Figure 94: Multiple cementation glazed scarabs - infiltrated with LUDOX solution (size approx. 65 x 35 x 35mm) [Photo by K. Nash]
Figure 95: Multiple hippo statuettes – infiltrated with LUDOX solution (size approx. 70mm x 20 x 30mm) [Photo by K. Nash]
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Summary of development work Materials and processes for a cementation glazed 3D printing technique have been developed through this practical enquiry.
Initial tests focused on establishing basic body and glaze compositions, to enable the first 3D printing trials to commence. Small, simple objects were fabricated during this early stage of development and the glaze powder composition was improved to increase its friability and reduce adhesion between the glaze powder and the object. Trials that aimed to assess the potential for expanding the colour pallet of cementation glazed objects were undertaken. Some potential for further development in this area was identified, however this was not pursued any further within the present research due to time restrictions. Gradient kiln testing revealed the optimum peak firing temperature for this process and also provided some information relating to the fired shrinkage of objects at these different temperatures. Object development trials revealed a lack of fired body strength which would make the process very limiting in terms of object shape and size. Several approaches to improving strength were investigated, two of which (bisque firing and infiltration) proved very successful in improving overall body strength, reducing glaze powder adhesion and providing a greater degree of consistency in terms of the surface smoothness and glaze appearance. Bisque firing objects also proved to be beneficial in controlling the glass transformation property that is characteristic to this process.
This glass transformation characteristic is an appealing property of the cementation process. A general characteristic of ceramic powder binder printing is a porous body structure which results in low object density and poor strength. The potential to transform a porous structure to a vitreous one has the overall effect of improving both strength and density in one go. However, the fact that this glass transformation property only occurs at thin sections, means that this property will not occur for larger, thicker objects, and so remains limited to fine, delicate objects.
A different approach to producing a more vitrified composition is explored in the next (final) trial. Instead of taking inspiration from traditional faience compositions, a different self-glazing ceramic was explored. Parian is a highly vitrified ceramic that was developed around 1845 by the Staffordshire pottery manufacture, Mintons. Favoured for its marble- like appearance it was named after Paros, the Greek Island known for its fine textured white Parian marble [Parian Ware, 2016]. It has an attractive pearlescent semi-gloss surface and is homogenous throughout. Like traditional faience, Parian is a single fire material
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however the firing temperature is significantly higher than at around 1200ᵒC. A typical Parian body is composed of china clay (30%) ball clay (10%) and feldspar (60%) and is generally regarded as being of medium to low plasticity.
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Trial 11: Parian body for ceramic powder binder printing This final trial explores the potential for producing 3D printed ceramics with improved strength and density due to a greater degree of vitrification in the body and an attractive semi-glossy finish. A typical Parian body [T. Buck, 2001] was used as a starting point for this trail and was adapted for powder binder printing. The highly plastic ball clay component was removed and the china clay component was reduced in order to minimise any layer shifting within the build. Molochite was added to recipe as this component is essentially calcined china clay that is no longer plastic, but has the same minerology as china clay. Instead of feldspar, Nepheline Syenite was used, as it has a lower thermal expansion and melting point than feldspar’s and is also favourable for its whiteness. Maltodextrin was used as an essential component for the powder binder process, as is shown as a percentage addition due to its complete combustion during the firing and absence in the finial composition.
3DP Parian Body Molochite – 13% China clay – 17% Nepheline Syenite – 70% Maltodextrin– 12.5% (addition)
Equipment and material preparation A 310 Plus, powder binder printer supplied by Z Corporation (now owned by 3D Systems) was used to fabricate objects in the Parian-like body for this process. The Parian body was prepared in the same way as the cementation body detailed at the start of this chapter - i.e. the ingredients were combined and mixed in a paint shaker for 12 minutes with metal mixing stars. The mixed powder was then sieved through a through a 149µm mesh sieve to remove coarse particles.
Three objects were 3D printed, dried for 48 hours and then fired to 1190°C.
Tile approx. size: 30 x 30 x 6mm
Donkey Bead approx. size: 25 x 25 x 25mm
Scarab approx. size: 30 x 15 x 7mm
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Figure 96: 3D printed Parian body unfired (background) and fired (foreground) [Photo by K. Nash]
Results The results of this trial showed that there is significant potential for further development into this body type for ceramic powder binder 3D printing. The three objects used in this trial were all successfully built and fired with an attractive pearlescent, semi-gloss surface. A significant shrinkage was observed (around 35%) which could potentially be reduced through the addition of some non-plastic fillers (such as silica) and exploring other fluxes (or a combination of fluxes) to produce a composition with a lower fired contraction overall. This high shrinkage is however a reflection on the improved density of these objects compared to other 3D printed ceramic compositions, and therefore it should be expected that a body type such as this should have a relatively high shrinkage if it is to be denser and less porous. Some distortion can be viewed on the 3D printed tile, however the other two objects display no visibly detectable distortion. It is thought that firing objects in a saggar and surrounded them with support media such as alumina bubbles could potential reduce distortion incurred during the firing.
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Chapter summary This chapter has presented key stages from the cementation body and glaze development work conducted within this research. The capabilities and limitations of this work have been investigated and some of the challenges have been overcome, such as improving the glaze appearance and minimising the degree of glaze powder adhesion. The fragility of the cementation body was found to be the main limiting factor of this process, however body strength was significantly improved by bisque firing 1190°C and infiltrating the body with colloidal silica (LUDOX). The glass transformation property unique to the cementation process was capable of increasing the density and strength of certain parts of the body. This finding led to a preliminary investigation into a vitrified, dense ceramic composition for ceramic powder binder printing that could potentially overcome some of the main limiting factors inherent to this 3D printing process, acting as a springboard for further research.
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