Thursday 21 February 2013

First Bisque Firing

Some little oxide/slip test tiles drying ready for the first bisque.

A few weeks ago I did my first bisque fire in my new kiln! What a big day. Although it actually it took three days…


Packing the kiln wasn’t as easy as it should have been. If I had to make a complaint about my (otherwise lovely!) new kiln, it would be the placing of the thermocouple.

The thermocouple is the device that records the temperature inside the kiln. It sticks out beyond the inside kiln wall (which is normal) but for some reason it’s been placed quite low down (where ‘logic’ would suggest it should be placed at least a third or half way up). Well anyway, the really annoying thing is the positioning is in EXACTLY the most inconvenient place for putting shelves!

Kiln shelves or ‘batts’ are supposed to be positioned away from the thermocouple – at least 2.5 cm above or below - because if placed too close they can damage it when they shrink and expand. Shelves also interfere with the temperature reading because they retain cold and heat more than their surroundings.

Shelf supports and three of the four shelves packed. The offending thermocouple can be seen in the top right photo.

Now here’s the thing. The shelf supports I bought as a ‘kiln set’ come in only two sizes: one short and one a bit taller. They can be stacked on top of each other to get different heights between shelves. So far so good. I currently have only four shelves (one of which is slightly thinner than the others!) and it was recommended to me not to exceed four because of the sheer weight of stuff in a firing. Shelves also have to be placed in a way to give pots enough room 'above their heads' so to speak. Also, for my first bisque I only had enough pots to just fill a kiln: and most of these were fairly similar heights...

Anyway, long story short and taking into account all these little variables (not to mention how heavy and awkward everything is!), let’s just say that Maths, Physics and the Universe were against me: because WHICHEVER combination I tried I could not get my first shelf packed without that infuriating thermocouple getting in the way! Either I had to make a tiny, low shelf at the bottom that could only take flat pieces (which would leave half the kiln empty above the top shelf) or I had to make a ‘huge’ gap on the bottom shelf that teetered on a long column of supports.

In the end I packed the kiln TWICE in two totally different ways, eventually plumping for the ‘huge teetering’ design - even though it made me feel really nervous because of all the weight I had to stack on top. I can only conclude that manufacturers never use the stuff they make…


I can tell you that I was way too tired to turn the thing on after all that! So I just closed the lid and decided to get up first thing next day to do the firing. Being a brand new kiln, it’s better to kiln-sit a first bisque to see how it behaves; which is why I decided to fire during the daytime (rather than put the kiln on overnight for a cheaper electricity rate). You can imagine that most of the day was spent popping in and out of the ‘kiln room’ to check the temperature and see if I’d programmed the controller properly.

Bungs and temperature on control panel (not long after end of firing).

At around 500ºC to 600ºC is the usual time for ‘putting in the bungs’. There are three holes in my kiln: two are vents and the third is a chimney (which I don’t use). These holes let out the steam generated from the last remaining bits of water being burnt off from inside the pots.

My kiln came with two ceramic bungs to plug up the holes, but I had to make a third one from an old piece of firebrick to block the chimney vent. I picked up a tip off the internet (or read it somewhere) that placing a mirror over a vent will help you to check if all the moisture has gone. Well I did this and I was still getting LOADS of condensation even after 700ºC.

Maybe this was because one of the shelves was new and hadn’t been ‘first fired’ yet. Or maybe there was just loads of moisture in the pots or generally in the kiln room (it had rained forever in the weeks beforehand). As this was my first bisque I really didn’t want to risk pots exploding everywhere so I waited until 770ºC when there was absolutely no steam at all before I put in all the bungs.

For anyone interested, my firing schedule was as follows:

First Ramp: 100ºC/hour to top temp 500ºC
Second Ramp: 200ºC/hour to top temp 900ºC
Third Ramp: 100ºC/hour to top temp 1000ºC

In other words I fired up to 1000ºC in 8 hours. Started at 7am and ended (everything switched off) by 3pm.


I thought I would be nervous, impatient or over-excited waiting for the kiln to cool. But by the third day I was pretty blasé about the whole thing: probably because I felt exhausted! Also I suppose I knew there was nothing I could do about it but wait and see.

I left the kiln to cool for exactly 24 hours: so at 3pm I checked the temperature. It was 60ºC - which is easily cool enough to risk cracking the kiln!  So I opened the lid…

Open sesame - top shelf of bisque pots.

And success! The top shelf looked perfect. All the little pink pots looked back up at me, all intact, and all looking like they were supposed to look. There was a bit of a crack in the kiln lid however and some kiln brick had dusted down onto one of my pots. This may prove annoying in the future come glaze firing…however, otherwise all looked well.

I emptied the kiln in stages, letting each shelf of pots cool for about an hour in-between, as they were still pretty hot the further down I went. I was also feeling a bit tired and wanted to unload carefully in case I dropped anything or did my back in leaning over the kiln! So by the time I had everything out it was getting dark.

I’m happy to say ALL my pots and every single piece I put in there (58 pieces in total) came out looking and feeling perfect – no blowouts or breakages at all – which is excellent news. Since then I’ve been glazing them too and they’ve all been soaking up the glazes properly. I’ve yet to see the results of a glaze firing, but all in all I’m very happy with my first bisque. And aside from the ridiculously annoying thermocouple and that little crack appearing, I’m really pleased with my kiln so far too.