Annealing

How long to hold annealing temperatures?  Kiln annealing and precipitation hardening.

Question

I just watched your video – https://www.youtube.com/watch?v=T2b4dPWUHLM – and am shocked at how quick your silver annealing process is. So far, I have learned that once silver is brought up to a dusty pink color, it needs to be held at that temperature – without letting it get to glowing – for about a minute. Your method doesn’t seem to require holding at a temperature for any length of time. I am excited to try it out later at the studio. How/why does it work, without holding it at annealing temp for a minute? Based on what I know (which isn’t much, lol), it doesn’t seem like it should be annealed yet.

Answer

I’ve been reading this, highly technical and often, over-my-head book:  Introduction to Precious Metals:  Metallurgy for Jewelers and Silversmiths by Mark Grimwade.  Ideally, for maximum softness (56 – 66 HV), sterling silver should be kiln annealed, at 730°C (1346° F) for 30 minutes and then immediately quenched. But, you can also anneal with a torch.  For torch annealing, a minute or two or three(!) is fine. The danger in holding it for 5 minutes is that you can over-anneal causing the formation of unwanted beta-crystals.  As the temperature is hotter with the torch, the time must then be shorter.

When torch annealing, the silver should be at 649° C (1,100° – 1,200° F).  It should be dark red – not orange.   Anneal in a darkened room to see the color change.  Immediate quenching is required to avoid the formation of unwanted beta crystals at the atomic level.  It’s all very complicated with alpha-phase crystallization and beta-phase crystallization.
Along those lines are precipitation hardening, aka: ageing and solution treatment.  sterling can be hardened by kiln heating the metal to 300°C (572°F) for one hour or 280°C for a couple of hours.  This process raises the hardness to 140 HV. The highest level of hardness is: 180HV.
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