INTERFACE LAYERS of Sn-Ag-Cu-S NIELLI INLAY in BRASS

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INTERFACE LAYERS OF Sn-Ag-Cu-S NIELLI INLAY IN BRASS ALLOYS Kageeporn Wongpreedee ,1,* Pathra Srisukho,2 Nipawan Kaimuk,1 Passaraporn Sae-ung,1 Suthasinee Settha-o-larn 1 1Gems and Jewelry program, Department of General science, Faculty of Science, Srinakharinwirot University, Bangkok, Thailand 2Faculty of Gemological Science and Applied Arts, Rambhai Barni Rajabhat University, Chantaburi, Thailand *e-mail: [email protected]

Abstract: Silver nielloware is an art-craft of processor Thais represented the wisdom of Thai culture. However, less people know recently because of the complexity of processes, toxic, and high price of silver. This work is to study all factors and technique suitable for niello black bar inlaying in the grooves of brass. The purpose of this study is to study the dimensions of groove and shape to inlay niello bar in it. The size and shape of groove was prepared by CNC and lost wax casting techniques. Various composition of brass and alloy addition is manipulated. The results show the smallest size of groove with any shape give the best result. Problems of niello inlaying in brass is the white spot areas in niello black bar filling in the grooves. The composition of white areas characterize by element mapping by EDS reveals that mainly reason is probably from Sn and Cu. High concentration copper with Tin base shows better interface layer comparing to common jewelry brass

Introduction: Nielloware is a valuable art and craft of Thailand since the early Ayutthaya period. Nilloware is made from silverware with the grooved texture filling with niello black bar. The process of art as an asset of Thai Wisdom required long hours of handmade. Historically, the black filling is called niello bar mainly composing of silver, lead, copper and sulfur [1]. Lead is a toxic substance that is harmful to human health and the environment. Due to this harmful reason, it is hard to export the product outside country and new generation of silversmith rarely getting into the process. Recently, it is firstly developed a innovated bar of filling as the new type of niello bar called "Nielle bar without lead"[2, 3]. The bar has the properties of glossy black base and good adhesions to the metal, which is not much different properties from traditional bar. From that pioneer research, it has been trial on various base alloys such as sterling silver, pure silver, and brass. The results seem to be normal obtaining desired properties of glossy, good adhesion, black color and less porosity except on brass bases. The more research is needed to be revealed the causes of problems. Therefore, this paper is to study the filling of niello bar in grooved made by CNC technology and on brass base at various compositions. The study of filling niello bar is how filling fit in various shape and size by CNC technology and what effects of interface between brass and niello bar.

Methodology: The experiments were separated to two sections. The first one is to study on various groove and shapes to be filled by the bars. The grooved samples were prepared by computer numerical control (CNC) at various size shapes of 9 models as shown in Figure 1. All size and shape would be waxed casted by centrifugal casting techniques to obtain all desired samples. The second part is to study interface of different brass bases. Two compositions of brass are 83.43 wt% Cu-9.47 wt% Zn-7.09 wt% Sn, called “high concentration Cu brass with Tin”, and 61.27 wt% Cu-38.73 wt% Zn, called “common jewelry brass”. The composition of brass is measured by XRF after casting. The process of filling Nielli Inlay bar which would be called niello black bar with the composition of 38 wt%Sn-11 wt%Ag -31 wt%Cu-20 wt%S is to fill in brass bases at 500 Celsius for 5 samples on each composition. All samples were mounted and observed using optical spectroscopy. The

© 38th Congress on Science and Technology of Thailand (Full paper) D_D0071 2 microstructure were then structured by Scanning Electron Microscope (SEM) and Energy Dispersive spectroscopy (EDS) in order to study the phase composition, porosity and grain sizes.

Grooving pattern Ratio (Width:Depth) Width*Length*Depth (mm) 3:1 1.5*10*0.5 6:1 3*10*0.5 12:1 6*10*0.5 3:1 1.5*10*0.5 6:1 3*10*0.5 12:1 6*10*0.5 3:1 1.5*10*0.5 6:1 3*10*0.5 12:1 6*10*0.5 Figure 1. size and dimension of grooving pattern

Results, Discussion: The results were determined into two aspects. The first result was performed by filling black niello inlay into the different grooves from small size to large one as indicated in Figure 1. The results show that all groove size and shape can be filled nicely. It is note that big size of groove required more time to fill in due to the bigger volume size. Therefore, the possibilities to observe porosity in the bigger size are higher. The second scenario is to observe the results of filling niello black bar in the groove at different kinds of brass. The results shows that niello inlay can be filled but it shows white spots on both kinds of brass bases and showed slightly porous as shown in Figure 2. The porosities in brass with composition of and common jewelry brass is revealed more than high concentration Cu brass with Tin as shown in Figure 3.

Figure 2. Niello black bar that filled the black lead on high concentration Cu brass with Tin (left) common jewelry brass. (Right)

Type of brass base Cross section Longitudinal Results alloys high concentration Cu White spot area brass with Tin and porosity (83.43 wt% Cu-9.47 wt% Zn-7.09 wt% Sn)

common jewelry brass White spot area (61.27 wt% Cu-38.73 and high porosity wt% Zn)

Figure 3. Optical microscope of two different alloys

The microstructure images in Figure 4 were taken by SEM and analyzed with EDS techniques. In the area of white spot in area B, it found that there is no Sulfur as shown in Figure 4a). It is revealed high concentration of Copper and Sulfur in that white area. Another interesting area is the interface area between white spot and interface of high concentration copper with Tin base. It is shown the migration of Zinc in D area indicated in Figure 4.

White spot

a) b) Figure 4. SEM microstructure of niello black bar with high concentration Cu brass with Tin (83.43 wt% Cu-9.47 wt% Zn-7.09 wt% Sn ) (a) shows white area of 38.47 wt%Sn-39.94 wt%Ag- 21.59 wt%Cu , (b) shows various composition of points. Point A 44.9 wt%Sn-1.43 wt%Ag-44.9 wt%Cu-10.38 wt%Zn-22.04 wt%S, point B 38.77 wt%Sn-4.8 wt%Ag-56.43 wt%Cu, point C 18.37 wt%Sn-34.39 wt%Ag-36.7 wt%Cu-10.54 wt%S, point D 26.25 wt%Sn-16.17 wt%Ag-35.04 wt%Cu-6.61 wt%Zn-15.93 wt%S

In the area of A and C, it shows the same ratio of Copper and Tin. The different of A and C is in area C shows higher concentration of Silver. This is explained why area C shows brighter scattering of an SEM image comparing to area A. In Figure 5, it shows the sample of common jewelry brass interface. All the concentrations of each element are summarized in Figure 6. It is observed that there is no interlayer interface unlike high concentration Copper with Tin base. Moreover, there is a crack along interlayer starting from interface of base and niello black bar to the top surface of its filling niello bar sample. This can be probably concluded that Sn is played an important roles of interlayer causing the adherence of the filling niello black base and the base. Another point of view of microstructure observation is to observe the scattering and combining all elements in the niello black bar. Figure 7 shows the mapping technique to see how white spot is arranged. It is confirmed that the white spot mainly arises from the combination of copper and tin. The segregation of sulfur can be combined with other elements revealing darker area of filling black niello bar.

Figure 5. SEM microstructure of common jewelry brass (61.27 wt% Cu-38.73 wt% Zn) point A 39.21 wt% Sn-14.35 wt%Ag-23.5 wt%Cu-22.93 wt%S, point B 64.31 wt%Cu-35.69 wt%Zn

Sn Ag Cu Zn S high concentration Cu brass with Tin (Figure 4) A 21.25 1.43 44.9 10.38 22.04 B (White spot) 38.77 4.8 56.43 C 18.37 34.39 36.7 10.54 D (Brass 26.25 16.17 35.04 6.61 15.93 interface) common brass (Figure 5) A 39.21 14.35 23.5 22.93 B 64.31 35.69 Figure 6 Shows the summary of all concentration on each spot observing by EDS

(A) (B) (C) (D) Figure 7. Element mapping of focus Nielli Inlay in brass alloy 2 (A)Ag, (B)Cu, (C)S, (D)Sn Nielli-Inlay alloys.

Conclusion: It is concluded that the big groove tent to be likely shown more porosities and white spot area due to the difficulties of filling niello black bar in the grooves. For the shape, there is no effect of shape. High concentration copper with Tin base shows better interface layer comparing to common jewelry brass. In the other way, it observed the crack between common jewelry brass and niello bar. It might be due to the higher concentration of tin making interlayer enhancing the adherance between filling and brass base (High concentration copper with Tin). White spot area is composed of the combination of copper and tin, mainly.

References: 1. Development of Business Development,Ministry of Commerce. Niello 2010:44. 2. K.Wongpreedee, P. Srirukho, P. Ruethaithananon, S.Harbantan, S.Kedsin, Black Nielloware without Lead, Thai Patent number, 1101001747, Sep 2011 (patent pending) 3. Srisukho P, Ruethaitananon P, Wongpreedee K. Microstructure and segregation of Sn-Ag- Cu-S Nielli-Inlay alloys for jewelry industry: 37th Congress on Science and Technology of Thailand 2011

Acknowledgements: The authors would like to gratitude all undergraduate and graduate students who are energetically research in this topics. All support work from Panphot Ruethaitananon, Siam Harbantan and Suratee Kedsin from department of General Science, Faculty of Science, Srinakharinwirot University. Special thanks to Assoc.Prof.Kunyada Anuwong for fruitful discussion. Lastly, thank you Ms.Pannarat Sornprachoom for the inspiration of the series of Nielloware work.

Keywords: Nielli Inlay, Nielloware, brass, interface