changing firing temps and time to produce a strong

[leezimmerley]

I was looking at the Art Jewelry magazine discussion forum, and one contributor posted a chart for PMC that shows the relationship between higher temps, length of firing time, and hardness.  I would like to find out - from very experienced Art Clay artists - how this applies to Art Clay 650.  I currently produce about 70 pieces of Art Clay jewelry in my line, and am still having trouble with strength in some pieces.  I don't subscribe to the idea that pieces MUST be at least 3 cards thick, necessarily, but I have had returns when customers try to reshape pieces, snapping them in half in the process. I use a Paragon kiln, and fire about 100 pieces at a time these days.  (I can't even imagine NOT having a kiln (using a torch? or a Speed FirE Cone - burned throug two of those in a month!), after being in business for over a year!

[Metalman]

My apologies regarding your question. I missed it in the system.
I went over to the Art Jewelry for a quick look but did not find the post you mentioned in a fast run at it.
Send me the link when you get a chance. If I look at that data , I may be able to address some specifics.

I have not seen data like you are requesting so far from Art Clay.   Reshaping of fired metal clays is problematic irregardless of thickness.  This is due to the structure that is formed as the particles 'sinter'.  This is, in general, true of all sintered materials.

With regard to thickness, I use a variety of strategies to help in this regard. Sometimes I thicken the edges and perhaps have something like veins running through the piece to lend strength and allow the in between ares to be thinner. Sometimes I build an underling structural form out of fine silver or Argentium wire to give strength where I need it but allow for less clay in the 'webbing' areas.  

The way I work, I frequently end up firing pieces several times - I believe the multiple firing adds strength.  From your description, it does not sound like this would work for you.  
You mentioned firing 100 pieces at once. That's great but having a number of shelves in your kiln will affect the firing.  In this case, I would recommend longer times to ensure that the entire batch of pieces gets to the right temperature for the required length of time.  This would also be a place where you might use some of the higher firing ranges, with an extended firing time to confirm that the proper sintering has occured.  

Why - you may ask as well as how do I know?
The shelves and your pieces work as insulation barriers in the kiln when you are firing - this means you will need a longer soak time to get full firing throughout the kiln. I have been using my Paragon SC2 for small scale glass fusing frequently.  I have layered in 2 or 3 shelves and done the firings - I have been amazed to the difference in the glass melt from the outside top edge of a shelf and the center , middle of a shelf. I would estimate as much as a 100°F temperature difference looking the fuse of the glass.  The same would apply to your metal clay firings.

Back to thickness-  for a press molded/stamped style object with no thicker edges or strengthening aspects of design, I would say 3 cards [.75 mm] would be a minimum, I usually use 4 for this type of object.

Again, my apologies for being slow in replying. Keep me posted on if this helps or additional questions.

[leezimmerley]

Thanks, Kurt, for looking into this.  I also emailed Mike Wilk at Art Clay, and received a similar response.  They do not, at this time, have the same information relative to hardness and firing times that PMC puts out, so I am working with that information and applying it to Art Clay.  I have found that firing  for an hour at 1450, for example, reduces the size of the piece by about 5%.   I should have realized that thee would be a heat discrepancy when adding a shelf (I've done pottery before) so I am upping my temps to about 1350, and firing for an hour.  I agree that bending is still out of the question - but I can't seem to get my retailers to convince their customers of that.  Adding another card probably won't end it, either.  

The bigger problem is making sure that people realize that the product is really, truly silver!  I've been questioned a lot about it.  It's getting old.

 '>

[decomama]

can any one tell me what happened to my art clay, I fired at
1400 for an hour my leaves were coated with 1mm of clay,
when I opened my kiln there was nothing there.

[Metalman]

Amazing!!

there was no ash or residues or anything?
What kind of kiln?  What kind of support did you use?
Was it Art Clay Silver 650 series or regular series clay or paste that you used?
YOu have my attention - send more details.
how many leaves?  How big?

[picturejewelry.com]

Did it blow up?