PMC porousity

Jewelry Discussion
1

Was: Tumbling PMC

Mary Ellen,

Once metal clay is fired, how is it still porous?

Hugs,
Terrie

2 
Once metal clay is fired, how is it still porous?

All PMC is porous when fired. This is because it is a powder and
there is space between the powder grains, before it is fired the
space is filled with the binder that makes it a “clay”. During firing
the binder vaporizes and the grains diffuse into each other.
Depending on which formula of PMC you use and what firing time and
temperature is chosen you will see the PMC achieve somewhere between
75% and 93% of theoretical density. To reach something closer to
theoretical density there must be a significant amount of pressure
applied to the item as it is being fired to help all the spaces
between the powder grains close up.

James Binnion
James Binnion Metal Arts

3 
Once metal clay is fired, how is it still porous?

It’s all to do with its density. By its very nature, it starts out
with a binder, which when fired, is burned off. This leaves tiny air
pockets - porosity. It is only heated to high enough temperatures for
the particles to fuse with each other. It is NOT heated high enough
for it to melt. The only way to eliminate the air pockets would be
for it to be heated to higher temperatures so that the particles
fused more closely but of course it would eventually melt if the
temperature reached high enough levels. This would obviously
obliterate any design you’d put into the metal clay. It’s a
compromise between retaining the design and removing the porosity.

Helen
UK

4

Since PMC is not milled like sheet or pulled like wire, it is less
dense than metal produced using either of those methods. It is not
truly porous, or exhibit the same uneven porosity as with cast
forms.

The lower density makes for lighter weight objects, which, of
course, means that larger designs are possible for jewelry, without
their becoming uncomfortable to wear.

Hope this helps,
Linda Kaye-Moses

5

Linda,

The lower density makes for lighter weight objects, which, of
course, means that larger designs are possible for jewelry,
without their becoming uncomfortable to wear.

Do you really notice a difference in weight? If “fully” fired at
1650F the claim is that it is 93% dense, not a lot of difference.

Jim

James Binnion
James Binnion Metal Arts