Wednesday, 19 October 2011

How it's made....bisque firing

The next stage in the making of a microdoll is the most critical.... the bisque firing, during which the soft-fired, relatively fragile castings are vitrified into porcelain.

The vitrification process literally melts the molecules in the greenware so that they fuse together, forming a strong, impermeable material with the properties of porcelain.  

During this high-temperature firing, each piece shrinks by up to 10% so careful loading of the kiln is very important.  I  first place a thin layer of firing sand on each shelf.  This serves two purposes.  Firstly the pieces can be slightly embedded in the sand which stops them rolling around, and secondly, as the pieces shrink during firing, they may 'grab' onto the shelf. The sand acts like millions of tiny ball bearings, moving under the pieces to prevent them sticking.
Every single piece must be placed carefully on the sand, not touching its neighbours.

This can take some time!

When the kiln is fully loaded, I set the programmer for a high-temperature porcelain firing. The programme varies according to how full the kiln is, the type of porcelain being fired (different flesh tones require slightly different firing schedules), and the age of the kiln elements.  Over time the elements deteriorate and the firing time gradually lengthens.  Eventually they will struggle to reach the top temperature, taking longer and longer to achieve a full bisque firing.  At that point it's time to replace them.

I replaced the elements on my kiln just a few months ago, and this firing is only the second full bisque firing since then so I have had to adjust the schedule to take into account the speedier firing time.

As it reaches the top temperature (1200-1215 degrees Celsius) the firing chamber glows white hot.  It's possible to see this through the gap between lid and kiln chamber.

I keep a close eye on things as the kiln nears the end of the firing, checking every 15 minutes to see the final few degrees temperature rise then the soak time, where the top temperature is maintained for a set period to fully mature the porcelain.

This last bisque firing took 6 hours.  Before I changed the elements a bisque firing was lasting 10-12 hours.  Once the firing is complete the programmer displays 'END' and switches off.  The temperature falls slowly over many hours.  I usually leave the kiln for at least 12 hours, by which time the internal temperature should be low enough to open the lid and check the firing.  If the lid is opened too soon, cold air rushes in to come in contact with the hot porcelain causing thermal shock.  This has never happened to me (thank goodness) but I understand that it is quite spectacular, as the porcelain pieces shatter, scattering like shrapnel, and possibly causing serious injuries.  One of the main advantages of my kiln is that even when the kiln is off, the internal temperature shows on the controller, so I always know exactly how hot it is.

However that is not always the case.

An underfire happens when the top temperature required to vitrify and mature the porcelain hasn't been reached.  As a result the porcelain has a chalky look and feels rough to the touch.
This is disappointing but not disastrous, as it can be re-fired to maturity.

An overfire however, is irrevocably catastrophic.  The porcelain will look shiny, often with tiny bubbles all over the surface.  Any flesh tones will have fired out to leave a ghostly look.
In the worst cases, the castings will completely collapse.

In order to avoid both of the above scenarios I keep a careful record of each firing, adjusting and fine-tuning subsequent firings as necessary.

And so to the next stage...... china painting



Elga said...

Now the fun begins!!!!

Seriously though, every time you talk about your kiln etc. the desire to make dolls again really rises its head. I hope my schedule will have time for it soon, right now though, I have to make the things that will bring the bacon home.

Robin said...

Another fascinating blog!
Thanks Sandra.