Tips and Information about making jewelry

With this blog, I hope to share my knowledge, successes, trials and errors, student's work, tips, and information about making jewelry.

Tuesday, September 17, 2013

Recap of My Sterling Silver Metal Clay Testing Part 1

I spent a year testing the PMC Sterling Silver metal clay and wanted to post the results on my own blog. So, here is the very first post.

Questions, questions, questions! It seems the more I ask questions and get answers, the more questions I have!
In the next six months I will be testing different aspects of the new PMC Sterling Silver metal clay. This is the first post. I want to explore various methods for firing this clay. There has been a lot of buzz about speeding up firing time, mostly because of classroom constraints. When it takes over 1 1/2 hour to fire the sterling silver clay, it makes it hard to have an afternoon class about this new clay.

Let's start with the recommended firing profile. When and where you have time, this is the way to go. According to Mitsubishi, the recommended firing schedule for PMC Sterling Silver is a two step process. Fire the dry clay in open air on a kiln shelf for 30 minutes at 1000˚F (538˚C) for at least 30 minutes, longer for thick pieces. The second step (after cooling the clay) is to transfer it to a stainless steel container, surround it with ½” of activated carbon (at least ¼” apart) and fire it again at 1500˚F (815C˚) for at least 30 minutes.

I fired some base-line test pieces using the recommended procedure. Phase-1 took 43 minutes. This includes the time for the kiln to heat up to temperature and the 30 minutes of soak time at 1000˚F (538˚C). Phase-2 lasted approximately 51 minutes (not including cool-down time). All my tests using the recommended firing procedure came out perfectly! I was able to dome a disc and bend a link without either of them breaking. With that said if you are working on a special piece and want to make sure it fires properly, use Mitsubishi’s recommended procedure.

With my testing, I wanted to find a quicker way of sintering the sterling silver clay. In my first set of tests, I used a 50g package of PMC Sterling Silver metal clay and created five identical oval pieces with their centers cut out (35mm x 20mm and 4 cards thick). I only changed the process for Phase-1 and tried a variety of methods, shown below. All pieces were fired together in Phase-2 using the recommended procedure. This is what I found.

Test 1- Light the clay on fire
In my personal blog, a reader commented that she used a torch for phase-1 (thanks PPennee), so I tried her technique. I lit the clay on fire with a torch and then pulled it way allowing the clay to burn. If it went out, I re-lit it until it no longer smoked when heated. It’s interesting; while holding the torch on the clay there is no smoke. Only after I remove the torch does the clay burn with a flame and smoke.

The Specs
Weight after complete firing (both phases): 1.6dwt
Measurements after complete firing: 30mm x 18mm
Phase-1 time: 1.5 minutes
Sintered: Yes – bent nearly in half.

Test 2- Hadar’s Method
Hadar Jacobson, who tests and makes base metal clays, designed a faster phase-1 for her clays. She uses a camping stove and a stainless steel bowl with activated carbon in it. She places the unfired dry clay on top of the activated carbon and then covers the bowl with a fiber kiln shelf that has a hole in its center. She fires it until there is no more smoke coming out of the bowl and then places it in the kiln for the second phase firing. In this instance, I used her phase-1 technique and then Mitsubishi’s phase-2 technique.

The Specs
Weight after complete firing (both phases): 1.65dwt
Measurements after complete firing: 29mm x 19mm
Phase-1 time: 10 minutes
Sintered: Yes – bent nearly in half.

Test 3 – Mitsubishi’s Procedure

The recommended procedure.

The Specs

Weight after complete firing (both phases): 1.70dwt
Measurements after complete firing: 29mm x 20mm
Phase-1 time: 40 minutes
Sintered: Yes – bent nearly in half.

Test 4 – Torch for 3 minutes
I heated the piece with a torch for 3 minutes. I heated it to a very dull red, like when annealing metal on a fire brick, allowed it to cool, and then completed the second phase.

The Specs

Weight after complete firing (both phases): 1.85dwt
Measurements after complete firing: 30mm x 19mm
Phase-1 time: 3 minutes
Sintered: Yes – bent part-way but broke sooner than the others.

More Questions
I had more questions after completing these tests. I made four discs 18 mm in diameter and 4 four cards thick.
  1. Does the clay shrink any more with longer firing?
    I fired disc #2 per Mitsubishi's instructions for 30 minutes. I then fired disc#3 per Mitsubishi's instructions for 2 hours in phase-2.
    Answer: both discs were the same size. They both measured 15.5mm after firing.
  2. Is the torch fired (in phase-1) clay less strong than the recommended firing technique?
    Answer: no.
    I re-did the test with disc #4 and successfully domed it. I did however anneal the disc after doming it a small amount and then proceeded to dome it more.
  3. What happens if I only fire the clay on the kiln shelf at 1500˚F (815C˚) for 1 hour?
    Answer: It doesn’t sinter (see Photo#1). The outside layer is metal and polished up to a silver shine with a brass brush but it broke very easily. The inside is dark gray and the outside is silver. I did learn though that it can still be sintered by completing the two phase process.

    Photo #1 left piece is not sintered.
  4. What happens if in phase-1 I fire the clay on the shelf for just 1 minute at 1000˚F (538˚C) and then complete the recommended phase-2 procedure?
    Answer: It doesn’t sinter.
  5. What happens if in phase-1 I fire the clay on the shelf for 10 minutes at 1000˚F (538˚C) and then complete the recommended phase-2 procedure?
    Answer: It sinters fully and is strong enough to be domed.
  6. What happens if I place a disc inside a screen under activated carbon and fire it at 1500˚F (815C˚) for 1 hour? Will the air inside the screen area be enough to allow it to sinter? (See Photo #2.)
    Answer: No, it doesn't sinter (see Photo #3 )

    Photo # 2

    Photo # 3
  7. If I am attaching two fired pieces together using oil slip can I bypass phase-1? If so, at what point does it not sinter?
    : It doesn’t sinter when fired at 1500˚F (815C˚) for 30 minutes. I fired three pieces separately and then used sterling silver oil slip putting them together. Additionally, I added syringe clay as a decoration and as reinforcement where the bail and the base connect. At first it looked like it sintered, the new clay shined up, but when I applied force it all fell apart. (see Photo # 4.) I then re-applied more oil slip and syringe clay, and fired it to the Mitsubishi’s recommended firing with phase-2 holding for 2 hours. It fired perfectly and doesn’t come apart with applied force. I will need to test this again with firing the piece for a longer time.
  8. Photo #4

    If you are working on a prize pieced, take the time and complete it using Mitsubishi’s recommended firing. If you are willing to cut some corners and time, then try some of the above tests. I wanted to make sure the tests were repeatable so I re-tested the pre-firing using the torch and made this pendant from test disc #4 and oval #3. It seems to be working. I can say that in a classroom situation, where time is short, I may use the quick firing using a torch for phase-1 as long as the work is not very large and complicated. I don’t want to risk harming someone else’s piece of art! I will also be sure that students know the recommended firing schedule and the benefits of sticking to it.

    I will continue testing multiple firing to see if longer firing allows the two fired pieces to adhere together without using phase-1 firing.


  1. Hi Janet- I sure am glad the torch firing worked for you! I'm with you, I won't do it on a masterpiece but it sure is a time saver for ring bands and basics. I really wanted to say thank you for doing the screen test! I had 2 pieces not sinter and I thought it was me- now I know that small amount of air space is the culprit.

  2. wow! tx so much for doing all the leg work! i do some metalsmithing and have been wanting to work with clay for years but no way could i afford starting another skill/technique at this time, but for reasons way too boring to mention, i am getting closer to taking the plunge in to clay, so a big mahalo to u and other artists for doing the tedious groundwork for me! your artwork is awesome, by the way! aloha, angi in hana

  3. Thank you Angi! Just understand, its not that much of a plunge. If you don't like what you made, you can grind it up with a coffee grinder, before firing, and re-hydrate it into new clay!
    Have fun!