Friday, 13 January 2017

Day 13 - Soft firing.....

When I've cast enough tiny dolls and toys to (hopefully) fill the kiln, or lost the will to live, whichever comes sooner, they will be set aside to air-dry completely for several days.  

The dry greenware is very fragile and can crumble to dust so must be handled very carefully.  However, the casting process leaves seam lines where the liquid porcelain slip seeps slightly into the gap between the two halves of the plaster mould.  These seam lines must be removed, along with any blemishes or pinholes on the surface of the casting.

It is possible to do this with a small piece of abrasive fabric (such as nylon net) but dry cleaning produces lots of dust, which contains silica, and is harmful to breathe in over a long period of time.  Also, as the greenware is so very fragile, it is extremely difficult to avoid breakages, especially when cleaning tiny pieces.

Years ago, I used to dry clean my castings, wearing a special mask to avoid breathing in the dust.  However the fine dust gets EVERYWHERE, so inevitably it is impossible to avoid inhaling it as it permeates clothing, fabrics, rugs etc.

Eventually, some bright spark discovered that if the greenware castings were fired to a very low temperature (approx 650 degrees, which is still much hotter than a domestic oven) any moisture remaining in the porcelain would be completely driven out, resulting in a casting which could be immersed in water without dissolving, but still be soft enough to be able to be cleaned.  And so the dust-free soft-cleaning procedure was born.

Soft firing does not vitrify the castings, so they can be loaded into the kiln with less precision than for a bisque firing.  They won't fuse together so the pieces can touch.  The castings will also become lighter and stronger, although they are still easy to break so require careful handling.

Here a selection of castings are placed on the bottom shelf in the kiln, with supports in place to support the second shelf. 

After the soft-firing is complete, and the kiln has cooled down, the castings are removed and safely stored in boxes.   Soft-fired castings can be stored indefinitely with less possibility of damage than unfired greenware.  

They are now ready for the next stage - soft-cleaning. 😩

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