Tuesday, April 02, 2013

It's more difficult than it looks: Cracking & Explosions

cracked pot, cracked ceramics photo crack4BIG_zps8ed14ad3.jpg

Next in my series of posts on "It's more difficult than it looks" comes something very natural to most clay -- cracking.

There are a few things that most people may not know about clay.  First is that "clay has memory"... meaning that the clay platelets that make up clay needs to be completely mixed evenly or else any settled clay may form cracks.  These cracks can form in the drying or more annoyingly in the firing stages.  Unevenness can happen if a piece is bumped, banged or pressed between the forming of the piece, all the way up until it is bisque fired.  Some clays are more forgiving, allowing slight mistakes in how it was handled, but others less plastic like porcelain, crack.

Cracks come in all shapes and sizes.  As with the example I found online, this one looks to be due to possible different thicknesses of the piece (uneven) and possibly a compression of clay wasn't done as well as it should have been.  These types of cracks aren't the type you can usually salvage.  If it cracked as it was drying, you can at least melt the piece down and reuse the clay.  If it's fired, if you have the right equipment, you can grind up the bisqued item into grog which can be used in clay to make the clay more workable.  Most people, however, just throw out those broken pieces.

I bring up explosions.  It's exactly what you'd think it is by the name.  Pots will explode in the kiln with much vigor. When air and moisture is trapped in the clay as it is fired in a kiln, the fast evaporation shocks the piece and will cause said explosion.  Explosions can be small or large enough that the shattered pieces can fly into other pieces, breaking them.  Pieces need to be bone dry before they are fired as well as a slow, even kiln firing.  Too fast of a ramp up can cause shock to the pieces.

So, sometimes someone else can damage your piece without them meaning to.  This just adds to the sometimes unpredictable nature of ceramics.

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