What are combustible cores, why would you want to use one, and what choices are there?
Cores are something you use as a support for your metal clay. There are two types: combustible and non-combustible.
Combustible cores include: Cork Clay, Wood Clay and paper, as these burn out completely. Non-combustible cores include plaster and Creative Paper Clay, these may crumble a bit, but do not burn out completely.
Combustible cores are necessary with Standard PMC, because there is a time during firing before sintering when the material could collapse. PMC+ and PMC3 do not require cores during firing.
Early articles on PMC show a wide variety of cores being used, including bread, shaped and then dipped in wax, and even Cheetos and Mini Wheats.
These days the most popular core products are Cork Clay, Wood Clay and Creative Paper Clay.
What are the combustible cores?
Cork Clay is made from ground up cork and some kind of binder. It’s from Japan. It’s amazingly workable and smooth, I love it.
Wood Clay is made from ground up wood scraps and some kind of binder, and is replacing Cork Clay. I attribute this to the change in the wine industry to rubber corks from you know, cork corks, but that’s just my personal opinion.
So when would you use Cork Clay or Wood Clay? When you need a core that will burn out completely, such as inside a bead.
What are the non-combustible cores?
Creative Paper Clay is an air dry modeling material used by doll makers and others. It contains volcanic ash and does not burn out completely.
Delight by the Creative Paper Clay people says on the package “do not bake, do not put in kiln.” Folks on Metal Clay Gallery say this is just to let people know that it’s an air dry clay. Still, I haven’t used it in the kiln, and wouldn’t, unless I checked with the company. Creative Paper Clay does not carry a similar warning. Maybe there is a reason.
Delight is super nice and smooth to work with, so while I wouldn’t use it for a core I would use it for other craft purposes.
When would you use Creative Paper Clay? When you want a custom shaped support for something, perhaps a bracelet or bracelet link, and it’s okay if the core doesn’t burn away completely.
Because Creative Paper Clay contains volcanic ash it does not burn away completely. If you use it inside a bead, it will take water and chipping away at the Creative Paper Clay with a sharp tool through the bead hole, to break it all up and shake it out. Not fun.
Where can you buy the new Wood Clay? From pretty much the supplier you used to get Cork Clay from. Creative Paper Clay is available locally at art supply stores such as Dick Blick and Pearl Art and Craft. I have had less luck finding it at craft stores.
Plaster and investment plaster are another type of non-combustible core. These are usually used for ring plugs and stone setting plugs, the latter being when you make a mold of a specific stone, usually a cabochon, then fill the mold with plaster, and use the plaster stone replica as a place holder during firing.
If you use investment plaster (high temperature plaster used in lost wax casting), I strongly recommend that you only use the silica free kind.
Why would you use a non-combustible core? To help make sure a piece made with Standard PMC retains its shape and does not slump in firing. The newer generation clays do not require support in the same way that the first generation clays (such as Standard PMC) did. Still, I usually don’t want to take a chance and never seem to have the time to experiment to see just how little support is needed. I admit, I over support things.
Also, if you’re making a hollow bead and want to make it completely in the fresh clay stage (i.e. not by assembling leather hard pieces) then it’s just plain easier to hold on to if you have a core inside.
We’ve covered the types of cores and that some are combustible and some are non-combustible. Why would you use each one? Use Creative Paper Clay for beads made of PMC Standard. Use Cork Clay, cereal, or whatever you like for PMC+ and PMC3, because the purpose of the core in that case is only to make the clay easier to work with in the clay stage.
A few important points about Cork Clay — it must be completely dry before you cover the Cork Clay core with metal clay. Allow it to dry completely, then coat it with glue (basic Elmer’s School Glue will do), this gives the metal clay something to grip on to and keeps it from falling off as you assemble your bead.
If the Cork Clay is not completely dry before firing, the moisture in the Cork Clay will turn to steam in the kiln and burst through the metal clay, creating at the worst, little volcano shapes.
I just thought of one more type of combustible core — the tiny items used to hold open a bail during forming. These are needed more during forming than in firing, yet sometimes they are impossible to remove, so it’s good if the item is safe to burn out.
Common items to use in making a bail are tiny noodles, and beverage stir straws. The noodles are called tubetti and are small tube pasta. Pasta is okay to burn out, straws are not. Unfortunately, sometimes when you try to remove the straw, the metal clay breaks. Oiling the straw first can help with this.
As an alternative, you can use a rolled up bit of paper, made into a tube.
Thanks to Celie Fago for suggesting I write a blog post on this topic.
(c) 2007 Elaine D. Luther All Rights Reserved