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Chapter 5: Practical Experiments Into Ceramic Powder Binder Printing and Cementation Glazing

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Chapter 5: Practical Experiments Into Ceramic Powder Binder Printing and Cementation Glazing 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. 106 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. 108 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 109 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
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