In my last blog I tested temperature hardening sterling silver. It worked great! (For more information on temperature hardening, here is a link to that blog.) After completing the testing, a nagging question hit me. At what temperature does sterling silver become brittle? We heat it all the time with a torch, but the constant heat in a kiln must change its properties, making it brittle. We have all been told not to heat sterling silver in a kiln above 1300°F (704°C) but my last test proved that at 1346°F (730°C) it was still pliable.
All of the tests were done with new scrap sterling silver sheet. I didn't want to keep using the same pieces as that could taint the testing.
First, I wanted to see if something about having the sterling silver embedded in metal clay caused a problem with it being brittle when heated above 1300°F (704°C). I embedded sheet and wire in-between two layers of PMC3. I also placed a piece of sterling silver wire in the kiln with the other two test pieces. I heated the pieces in the kiln at 1346°F (730°C) for an hour. I quenched each piece when the kiln's temperature reached below 500°F (260°C) All metal pieces are still pliable without breaking after bending 1-3 times, so it has nothing to do with the metal clay.
I raised the temperature to 1366°F (741°C) and heated sheet sterling silver for one hour. It passes the bend test after bending it 4 times.
I heated the sterling silver sheet to 1460°F (794°C) for one hour. It bent fine the first two times, but then broke on the third bend into two pieces.
I heated the sterling silver sheet to 1500°F (815°C) for one hour. It bent fine the first time but broke on the third bend.
I heated the sterling silver sheet to 1500°F (815°C) for two hours. It broke on the first bend.
My assumption is there isn't an exact temperature when the sterling becomes brittle. As the temperature goes up, the sterling becomes more brittle and the longer the metal is heated the higher the chance of it becoming brittle.
I heated the sterling silver sheet to 1346°F (730°C) and soaked the metal with heat for 2 hours to see if a longer exposure to this temperature would cause it to become brittle and it did.
In conclusion, heating sterling silver up to 1346°F (730°C) is about the highest heat recommended, and only heat it for a short amount of time.
Just after I finished writing this report, Linda Kaye-Moses emailed me with some great information. She uses the book, Theory and Practice of Goldsmithing translated to English by Charles Lewton-Brain as a reference book. She was kind enough to pass on this information.
Linda Kaye-Moses wrote:
"What you want to aim for as the final stage of working metal is fine grains, at the microscopic level. “Grain formation is only possible when the atoms are given enough movement energy in the form of an annealing heat so that they can leave their old position.” So, a work hardened piece must be annealed to return it to a more workable grain size. This temperature for sterling silver is around 200 degrees C (392 degrees F).
“If metal is heated for longer than is necessary, newly formed grains fuse together to form larger grains. The resulting large grains... affect malleability, so metal should not be annealed [heated] any longer than is absolutely necessary...Recrystallization structure [that is, the realignment of metal grain] is affected by annealing temperature...the minimum temperature required for the recrystallization differs for various metals. The more the temperature is raised above this minimum temperature, the faster the grains grow and eventually an enlargement of the entire structure occurs. Conditions that contribute to large grain formation are [pertinent to metal clay]: too slow heating when annealing; too high an annealing temperature; too much time at annealing temperature.”
Large grain structures in metal contribute to brittleness. “Large grained structures are unsuitable for mechanical deformation [like bending, forging, etc.] and have a tendency to crack.” When sterling silver is heated for long periods of time and at high temperatures (firing temperatures and durations for metal clay), it becomes brittle. “It is obviously desirable to...create the finest grain structure possible. To achieve this [pertinent to metal clay] heat it as rapidly as possible to the required temperature; Stop as soon as it is hot enough; do not prolong annealing time.”
“To age harden a silver-copper alloy, the metal is first annealed.” This is the first step you describe: Heat the sterling to 1292°F–1346°F (700°C–730°C) for 30–60 minutes. This annealing recrystallizes the metal, forming an internal small grain structure. “...then [the metal] is quenched...”, the second step you describe. This step makes the metal retain its small grain structure. The metal is then age-hardened as follows: It is “...heat-soaked (tempered) and allowed to cool slowly.” This last step is as you described...Heat the sterling again, this time to 572°F (300°C), holding at that temperature for 30–60 minutes.
It seems to me that the reason that ss metal clay becomes brittle, is that it is heated at too high a temp and for too long. Quenching does not alter the grain structure in this case. In fact it does very little to the metal.
Until next time, have fun claying around!
by Janet Alexander