Metal Clay Questions

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Breaking prongs and annealing fine silver.

Question

07/03/17

I was hoping you could help me out with some kiln annealing temps. I have had a couple students break their fine silver bezel prongs while setting their stones. I haven’t had this problem but am looking to increase their success rate and I was wondering if kiln annealing (many don’t do torch firing) is possible for fine silver. I have kiln annealed 960 blends at 1250 for 25 minutes but it seems that maybe I should be using a lower temp. The last firing for these pieces (they go through a couple firings) is 45 minutes at 1472F.

Answer

Hi!  I thought, for maximum strength, the clay should be fired at 1650 for two hours? After this temp and time, maximum sintering occurs.  

Also, I think your annealing temps are too high.  After the crystallization begins, in the metal, the crystals can start randomly growing if the heat gets too high.  This can cause an orange peel surface, when it is bent and an uneven, weakened crystalline structure as the larger crystals push against the regularly sized grains.  Try annealing for 30 minutes at 773°F.  I wouldn’t go higher than 900°F.  The 773°F is determined by finding the melting point in Kelvin and then dividing by 3.  It’s a rule of thumb number for a few of the pure metals. That said, the metal should be pretty soft when it comes out of the kiln, after firing.

An idea to try and solve the problem  is to create larger, thicker prongs (or whatever you are making) then thin AFTER they are set over the stone.  Escapement barrette files work great for this – especially if the edges are burnished. You could also switch to sterling metal clay for these components.

BTW, fine silver doesn’t need to be quenched. But, it is often quenched to stop the recrystallization process and avoid excess crystal growth.

Fine silver  cannot be age hardened because it doesn’t contain a secondary alloy.

Resources

Problems with firing white copper clay.

Question

I am having problems with white copper clay, after months of trying different things. I have been in contact with Bill Struve of Metal Clay Adventures. He has changed the firing schedule to 650 degrees for 30 min then high ramp to 1900 degrees for 2 hrs. This schedule has (so it seems) has stopped the cracking, but they aren’t shrinking completely and I tried bending one piece and it broke and was solid black in the center. Does that mean it didn’t sinter completely? Any advice? I hate to keep bothering Bill.

November, 2015 from Facebook.

Answer

I don’t use metal clay much anymore. But, when I was working with clay, I had a lot of problems with the base metal clays. I had the same problem with cracking and it being black in the center. It’s just not fired completely. I’d do a series of test pieces (of a similar mass to your finished piece) and keep track of what you are doing. Try firing longer and hotter. Run through various temperatures and times, firing one piece at 1925, the next at 1950 and see what happens to the clay. Record your results. That’s the best way to go as everyone’s kiln is different: Your 1900 degrees is not his 1900 degrees – yours might only be at 1825. To check: you can purchase a probe thermometer (the probe is sold separately) and verify the internal temperature of your kiln – while firing. I have one with a probe that I insert in the vent hole of my kiln. (Sounds like an alien abduction!)  Keep track of any differences and adjust your kiln accordingly. So, experiment – it’s so much easier and cheaper (in the long run) than constantly ruining your work. Good luck.