Tinning procedure for model samples

There are several methods used for the preparation of model tinned samples to evaluate the nature of intermetallic compounds formed. In the literature, methods include electro-deposition (Hedges, 1964; Fujiwara et al., 1980; ITRI, 1983; Price, 1983) and

vapour-Cu/IMCs/Sn structures. These methods can be controlled to produce repeatable tinning outcomes that improve quality control of samples, but do not necessarily resemble the product of traditional tinning methods used in antiquity. Previous experimental heritage tinning work includes tinning by dipping in molten tin and by wiping molten tin on a copper or bronze surface (Tylecote, 1985; Meeks, 1986). Tylecote (1985) immersed high tin (16-18wt% Sn) bronze tokens into molten tin from room temperature after having applied ZnCl paste flux on the bronze surface. Immersion at a range of temperatures/time sets resulted successfully in growth of a thin η-Cu6Sn5 (1µm after 1 second at 234°C) or thicker layer of η-Cu6Sn5 (6µm) present with underlying ε-Cu3Sn (1µm) at higher temperatures (30 sec at 285°C). Tinning by immersion is generally reported to produce relatively uniform layers (Lee and Duh, 1999; Lee and Chen, 2002; Yu et al., 2005; Wang et al., 2006). A systematic investigation by Meeks (1986) employed traditional tinning by fluxing with rosin and wiping tin on to a range of copper and bronze substrates (5wt%, 10wt% 20wt%Sn). This produced microstructures that were further investigated to generate examples of changes occurring during post-tinning heat treatments (or annealing) at 200, 250, 350, 450, 550 and 650°C for 2, 5 or 60 minutes (ibid.).

Experimental tinning in this work involved the tinning of pure copper and bronze (c.10wt%

Sn) samples using both dip-tinning and traditional wiping (Davies, 1882; Fuller, 1894: 20-21;

Politis, 2005; Panagiotis, 2007). Both copper and bronze samples were used to facilitate observation of intermetallic phase growth. Tin in the bronze affects the microstructural nature of IMCs due to differences in the diffusion of Cu and Sn in the bronze alloy as compared to pure copper. The dip-tinning method was employed to produce coatings on both sides of each sample to increase relative volume area and concentration of IMCs in the sample. This would permit evaluation of detection limits of IMC phases by neutron diffraction as the volume area of wipe-tinned coatings could be too thin for ToF-ND to detect. Annealing temperatures and time replicate Meeks’s (1986) tinning experiments to provide a platform for comparisons of microstructures developed.

Flux is critical for removing surface oxides and increasing wetability of the tin onto the warmed substrate (Subramanian and Lee, 2003). Inorganic zinc chloride based fluxes are commonly used by modern tinsmiths (Fuller, 1894; DKL Metals Ltd, 2006) (Politis, 2005;

Panagiotis, 2007)., but in industry rosin-type mildly activated fluxes (RMA) are preferred (Prakash and Sritharan, 2001; Elenco Electronics, 2002; Yu et al., 2005; Suh et al., 2008; Zou et al., 2008). High carbon concentrations at the Cu/IMC interface can develop as a result of

likely to have been used in antiquity and because it is reported to be the reference standard used in industry to evaluate performance of synthetic fluxes (Srivastava et al., 1989).

Materials used and tinning procedures were:

Substrates used:

(a) A pure copper (99.9% Cu) sheet of medium hardness was cut into 20x50x3.25mm tokens.

The outer surface of the copper was finely polished by the supplier.

(b) Bronze of c.10wt% Sn was prepared using pure copper (99.9 wt% Cu) and pure tin (99.75% Sn), supplied by Goodfellow Ltd. Metals were melted in a graphite crucible and poured into preheated flat graphite moulds (13x25mm); they were left to cool to room temperature. The bronze cast tokens were annealed at 550°C for 6 hours prior to tinning in an attempt to eliminate potential tin segregation developed during casting.

Wipe-tinning procedure for copper and bronze tokens:

1. The metal token was cleaned with 50% HCl acid (32% conc. stock solution) with cotton swabs;

2. the token was heated lightly over a yellow flame of a Bunsen burner;

3. pure rosin was applied on the heated metal surface using pliers and spread on the surface whilst keeping the token near the open flame;

4. a rod of pure metal tin was passed over the metal surface and melted immediately spreading over the metal with great fluidity;

5. the molten tin was immediately wiped over the metal surface using cotton pads and excess tin was removed;

6. the process was repeated (from step 2) until the surface of the token was covered with a layer of tin;

7. the token was placed on a heat-resistant mat to cool.

Dip-tinning procedure for copper samples only:

1-3 as wipe-tinning.

4. The token was held with pliers in liquefied tin within a ceramic crucible for approximately 5 seconds;

5. the token was removed from the molten tin and placed on its side to cool.

Two sets of each tinning method and substrate type where prepared prior to post-tinning

al., 1989) rather than by ultrasonic bath (Zou et al., 2008). Annealing took place at 200°C, 250°C, 350°C, 450°C and 550°C in an electric furnace. Different growth rates and mechanisms are involved at different activation energies (Laurila et al., 2005), but in general it is reported that longer aging times at the same temperature result in the growth of IMC layers (Lee and Duh, 1999). For this reason the tinned samples were annealed at each temperature for 5 or 60 minutes. Each tinned token was placed from room temperature into the furnace, which was already heated to the required temperature and then it was removed back to room temperature after 5 or 60 minutes (Table 3.8).

Experimental samples

Table 3.8: List of experimental tinned samples produced by wiping or dipping tin on copper and bronze substrates. Annealing temperature/time is shown per sample. Bronze substrates were annealed at 550°C for 6 hours prior to tinning.