Thursday, March 29, 2012

Sterling Silver Test 1- Firing the Clay

by Janet Alexander
Technical Advisor

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.


Cindy ~ said...

Excellent read!!

Lora Hart said...

I wonder... if all three pieces were sintered separately to begin with, whether you need the 2nd phase firing when joining at all? What about sintering separate elements and then joining with .999 oil slip/paste and syringe in the first phase only? If the .925 is cooked good, then you only need to cook the .999... right?

Lora Hart said...

Oh, and not so by the way, thanks for doing these experiments.

Anonymous said...

Bravo Janet! You are really leading here...and sharing your experience is *awesome*! Thank you...I'm still trying to find time to leverage all I've learned from you...but you're learning me faster 'n I can practice! :-) Laurie Trice

BJangled said...
This comment has been removed by the author.
Sharon Gillespie said...

Great read Janet. Will talk to you later. Thanks for doing these tests.

Janet Alexander said...

Hi Lora,

All three pieces were sintered separately, but putting together with the sterling silver clay in this test for a 30 minute firing didn't sinter. Next time I will try firing for 2 hours like Vickie Hallmark. In this test I was looking for a faster firing time.

If I join the separate fired pieces with PMC3 I only need a single firing. I have a ring I made with PMC3 and Sterling silver clay, combining them with the PMC3 slip. Since I had a stone in it, I fired it at 1110 for 1 hour.

Janet Alexander said...

Thanks Laurie! Its nice to see you here!

Anonymous said...

Thanks for sharing your findings!! -Gordon

Third Dimension Jewellery said...

Very interesting, thanks. A combination of test one and two works with Fastfire Bronzclay and the Speedfire Mini Electric kiln, too.

Janet Alexander said...

Thankyou Gordon, Sharon, and TDJ!.

PPennee said...

Thanks for doing all those extra experiments Janet- I am so glad the torch firing was successful. Now I have to try to see if I can torch fire joined pieces then fire to schedule. I have tried that with pmc3 oil paste and it worked- so maybe it will on simple pieces, but I agree- I sure wouldn't do it with a piece of art!

Janet Alexander said...

HI PPennee, let us know how it goes!

Raphsody said...

Hi Janet, Thank you for your blog and the article it is very useful, however there is something I need a bit of a clarification on. Could you please explain, what does " Fire the dry clay in open air on a kiln shelf for 30 minutes at 1000˚F (538˚C)" exactly mean? I am trying to do my first project but I am a bit confused by the "open air" Many thanks,Andrea.

Janet Alexander said...

Hi Andrea,
The sterling silver metal clay must be fired in two stages, with both stages using a kiln. The first stage is done by laying the dry clay on a kiln shelf in open air without using any carbon. Sometimes I will put it on top of a fiber blanket or in a bowl of vermiculite(for support). The dry clay is fired for 30 minutes at 1000˚F (open air). I wait for the kiln to cool, and then place the sterling silver clay in a stainless steel vessel with coconut carbon under and above it, read the manufacturer's instructions. Its then fired at 1500˚ F degrees for 1.5 hours. I hope this helps.

Martina Eiselein said...

Hi Janet, thanks for sharing your test! Can you tell me how much is the shrinkage rate of sterling clay?
Thank you in advance and best wishes Martina Eiselein

Amy Gervais said...

Hi Janet,

My timer didn't go off, and I've left my PMC sterling pieces for 45 minutes open air in my speed fire 1600 - the temperature definitely went above 1000 degrees. I am now completing the 2nd stage with carbon as usual.

Can you shed any light onto whether or not the overcook in the first stage may do any harm to my pieces? (some were fired and being pasted to other other pieces and some were greenware)

Thanks, this article was super helpful!


Janet Alexander said...

If the sterling silver clay was heated too hot, it may tarnish. This tarnish keeps the particles from fusing together in the second stage firing.
The best advice I can give you is to try it and see if the clay sinters.

Best of Luck!

Anonymous said...

Hi everyone, I just read that you can fire sterling silver clay only once if using magic carbon! apparently it allows enough o2 in?

Janet Alexander said...

Hi Anonymous,

I haven't had success at the one step firing others have talked about.

I have done the first burn off with a torch successfully and then fired it in the carbon. If using a torch, you just want to warm the clay up until it catches fire and then remove the torch. Don't heat it too hot or you tarnish the particles and it doesn't sinter correctly. This process only takes a few minutes.

Anyway, I warm it until it catches fire, let it burn, and then keep catching it on fire until it will no longer burns. Then its ready to put into the carbon for the 2nd firing.

Please note, Mitsubishi recommends firing the clay as stated in their directions.

kUrOhOsHi* said...

I was wondering if I can just use a butane torch on pmc sterling, because all i have is that, and I dont have a kiln or carbon

kUrOhOsHi* said...

I was wondering if I can just use a butane torch on pmc sterling, because all i have is that, and I dont have a kiln or carbon

Anonymous said...


I am working with PMC+ for the first time, and I have only fired it twice. In each case it did not turn silver. After firing it just looks white like clay... I fired it at 1650 degrees for 2 hours along with some glass on a clay mold. Any idea why it didn't turn silver??

Any help would be most appreciated.


Lora Hart said...

Sherry, the natural color of unpolished silver is white. When the metal comes out of the kiln, the metal crystals are standing on end, so when the light hits, it's reflecting the white part of the light spectrum. When you polish or burnish the metal, you flatten the crystals allowing the metal to show the silver. I suggest you join the FaceBook group Metal Clay Now so your questions are answered more quickly and by a variety of opinions. Thanks for finding the blog.