Saturday, 8 May 2010

Fruits of my labours.......

Following my marathon casting/soft-cleaning/kiln-loading exercise over recent weeks, here is a sight to gladden the eye of anyone who works with porcelain.

What you are looking at is the top shelf of three, following a bisque firing. All those tiny limbs, perfectly fired, represent the culmination of a month's work, and it doesn't stop there.

No by no nonny no.

On the bottom two shelves underneath are all the little bodies/heads, which will each require up to 4 subsequent china paint firings.

However, at this point, I am simply relieved that the bisque firing went well. I know of no ceramicist who doesn't experience a frisson of trepidation just before opening their kiln. There are many things which can go wrong, most of which I have experienced during the 25 years I've been working with porcelain.

  • Minor glitches might involve pieces having shifted around during the firing process. This is fairly unusual because the porcelain shrinks and pieces move further apart. However if pieces end up touching, they can fuse together. I implement an exclusion zone around the kiln after it's been loaded, to avoid anyone inadvertently knocking it slightly, and possibly jolting the carefully placed pieces inside. This, of course, is a hanging offence.
  • Underfire - this is very annoying, but fortunately rectifiable. If the temperature within the kiln doesn't reach the highest programmed point for whatever reason, the porcelain will not vitrify (mature) properly. This results in pieces which look 'chalky'. The solution is to refire and ensure that the top temperature is achieved.
  • Overfire - disastrous. I have only had this happen once, with a previous kiln which used a kiln sitter device to switch the kiln off when the top temperature was reached. One of the kiln shelves was resting against the kiln sitter (my fault entirely), so the sitter wasn't tripped off when it should have been. The temperature continued to rise to the maximum and stayed there for ages, until I noticed the unusual acrid smell emanating from the kiln. This is a Bad Thing. It doesn't do the kiln elements much good either. However the main victim is always the contents. Overfired porcelain is glassy and shiny. Instead of a lovely, healthy flesh colour, it turns deathly white. The surface is usually pitted, and there may be tiny bubbles, where the porcelain began to 'boil'. The only solution is to bin the lot. Devastating if it's taken weeks to cast enough dolls to fill the kiln.
  • Thermal shock. This can be scary. My previous kiln had various plugs and bungs to vent the kiln at different stages of firing. If the kiln is very hot, a blast of cold air into it can lead to thermal shock, which causes pieces of porcelain to explode, as very hot meets very cold. A bit like the Icelandic volcano's erupting magma meeting glacial ice. Kaboom! Obviously, explosions within a confined area, can cause a lot of damage, usually to the body of the kiln itself. I have only experienced one fairly minor example of this, early on in my firing career when I was still going through the 'let's try this and see what happens' phase, known and loved by all novices. I vented one of the plugs too soon, letting in very cold air, and could hear some alarming pops and cracks inside. Hours later, when the kiln was cool enough to open, and I lifted the lid, I discovered that a few bodies right by the vent had shattered, taking out some of their neighbours in the process. A porcelain version of 'friendly fire'. Luckily, apart from a few dents in the fire bricks lining the kiln wall, there was no damage to the elements.
These are just a few of the mishaps which can greet the anxious eyes of a dollmaker when opening the kiln after a full bisque firing. The lower temperature soft firings, and china paint firings, by comparison, are a piece of cake.

Speaking of which, I could just go a cup of tea and big bit of cake.

Hmmmm.... cake.

3 comments:

Betty said...

Thank you for sharing your thoughts on a porcelain kiln! I always wanted one but sadly not enough electricity. It looks really cool though :)

P.S. Your dollies and work are just lovely! Keep up the good work!

Maria Narbon said...

I understand perfectly.
But I have another problem I still do not found a slip without black spots!!!!
I am amazed at the amount of toys you can make in a month, I could not in two months, I'm getting old, ha, ha, ha!

pastmastery said...

Hope it was VIRTUAL cake Sandra.
;)
That's what I have to have nowadays!
Sue
x