Cleaning Ceramic Molds
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The very nature of this plaster is what makes it possible to make poured ceramics, in that moisture on one side draws through to the other side. This is how your greenware is formed.
By rapid extraction of water from the slip that is against the side of the mold, a thicker consistency begins. As this thicker consistency (less water) builds up against the surface of the mold, a shell is being created. When that shell is thick enough, excess slip is then removed (dumped out), leaving only that thicker layer lying against the surface of plaster.
The plaster continues to draw water through the body of the mold, causing the slip inside to become drier (stiffer).
Through the process of many pourings, sodium silicate begins to build up inside the channels running through the body of the mold. This is the reason why, after many pourings you will notice a white fuzz forming on the outside of the mold (the opposite side from where the slip touches the surface).
This white fuzz is actually sodium silicate. Despite the odd appearance, this is no cause for alarm. This is to be expected.
Many people mistakenly think it best to leach the mold, in order to keep it in good condition. The problem there is the fact that this can only cause damage.
Remember that the reason plaster makes our greenware for us, is channels running through the body of the mold. Leaching accelerates the wear in those channels, making them larger which will weaken the mold. The larger the holes running through your mold, the weaker it is. Makes sense, doesn't it?
The truth is, your mold will not harbor sodium silicate. Once there is a build up in the channel, each time you pour again, more moisture runs through the body of the mold, pushing last weeks sodium silicate through and replacing it with more.
What this means is, once the mold is saturated with sodium silicate, it will always be saturated but always being replaced with new. This is ok. Better to keep the process going than to try to rinse it out. For one thing, it will rapidly be replaced the next time you use it. What's more, completely saturating a mold with water simply wears out the channels and you don't want that.
So how does a body clean a mold? Clean off excess slip with a rubber mold knife, being careful not to gouge. That and sponge off the surface, if you want but I've never really been bothered by the outside appearance. All you're really concerned with is removing slip that has spilled down the sides, to prevent it from blocking off those channels. That's it.
If you've had a problem with a pouring and found yourself with torn pieces inside that just won't pull out (like in a tiny corner or nipple area), it's best to let it set up some more (shrinking) and try again.
Judy Sims