Thursday, 12 January 2017

Day 12 - Now for the science....

I use many different tools and pieces of equipment, but the one which is completely indispensable, and without which I simply couldn't carry on the business, is my electric kiln.  In simple terms, a kiln is a furnace, or oven for burning, baking or drying something, or more commonly for firing pottery.

This is the largest top-loading kiln I could find which would operate via a 13 amp standard UK electric plug and still reach a top temperature of 1260 degrees Celsius, the temperature required to vitrify porcelain.   The vitrification process uses heat to convert the chalky, fragile greenware into a glass-like substance - bisque porcelain. 
Although it isn't strictly speaking 'glass', if porcelain is held up to light, it should appear translucent.  

Inside you can see the thick, insulating kiln bricks, which help keep all the heat inside, and running in channels around the inside edge, the spiral, metal kiln elements, which glow white hot at their top temperature.  Within this firing chamber I can place up to four shelves, one on top of the other, separated by shelf props, in order to maximise the firing capacity.

This is only my second kiln in almost 30 years.   The first, which I had for 15 years, was a Cromartie Hobbytech 40, slightly smaller and a manual kiln.  When I decided that I wanted to upgrade to a computer controlled kiln, I was able to sell my old one for the same price as I bought it for. 

Cromartie kilns are THAT good.

Sadly, Cromartie, a UK firm based in the heart of the potteries district of Stoke-on-Trent are no longer making kilns, but due to their excellent build quality, existing Cromartie kilns will continue in use for many years to come.

My firing schedules consists of three different stages - soft firing, bisque firing and china paint firing, each of which has different ramp times, temperatures, soak times etc, which I have programmed into the computer controller.  Once it's set to fire, the computer takes over and regulates the whole process, which takes the guesswork out of things.  

However, skill and experience are required in order to get perfect firing results and I have  worked out my own set of firing programmes, gleaned from hundreds of firings over the past quarter of a century.  These programmes have to be adjusted according to the type of porcelain slip I'm using, the size of the castings and age of the kiln elements, among several other factors.

Having only recently replaced the kiln elements, I've been able to knock  several hours off a full bisque firing, as the new elements can reach the required top temperature much more quickly.  However this means that I have to reduce the soak times and adjust the top temperature to suit the tiny pieces I'm firing, otherwise I risk a catastrophic overfire.

It takes a LOT of dolls and toys to fill my kiln, which is why I have to spend several weeks casting so that I can hopefully fill at least three shelves with tiny doll parts.

Next up.... soft firing and cleaning. 

1 comment:

Kathy Moore said...

Sandra, I truly admire the talents and expertise you have amassed over the years.