Out of the Fire! A glaze recipe, and kiln stuff!

I know it is unfair of me, but..... I thought I would start this with some close up photos of a couple of the pots that we unloaded from the wood fired kiln yesterday.  You will just have to wade further through this post if you want to see the complete pot!  It has been a long time since I last fired the kiln, and I had forgotten just how beautiful a wood fired earthenware pot can be; and it was the textures, and the way the light twinkled and flashed off tiny iron crystals that had formed in the glaze, that immediately stole my heart away.  The simple combination of red clay, white slip, and honey coloured glaze worked so well, that there was really no need to search any further for bright blues, reds, oranges or greens!  The photos really give no more than a hint, as they lack the playful scintillation of light..., but they will have to do!

The Return of Wilson and Marti!

Those of you who have followed this blog for some time, may remember that about a year and a half ago we enjoyed a visit from Wilson and Marti from Dunedin, Florida. They not only lived at a place with the same name as our Dunedin, but Wilson was also working with crystalline glazed pottery.  Yesterday it was a great pleasure to be able to share the kiln opening with them as they once again visited this part of the world!

Man standing in kiln firebox... whilst onlookers speculate...!

My level of technology is probably well summed up by, "put your hand through the spy hole, and if the inside of the kiln doesn't burn you then it is ready to unload".  This is quite different to what Wilson is used to in Florida, and I think we both found the contrast really fun.  

Here we are again... I'm trying to look less tall than when we tried a similar photo a year and a half ago,
http://opopots.blogspot.co.nz/2011/01/good-things-come-to-those-who-wait.html

Conversation flowed all day and it was really hard to say goodbye to Marti and Wilson, they are really lovely people.

Anyway, back to the kiln...

Last post I left you with the kiln about half way through its firing, and Laura stoking.  Well, the firing went rather faster than planned.  I had my very inaccurate pyrometer probe measuring temperatures through a neat little hole that I drilled in the chamber roof with a brace and bit (a primitive hand operated tool that looks like the thing people started cars with, before starter motors were invented!). My pyrometer always under reads by a considerable amount, especially at the higher end of its scale, so I really only use it to help me judge if the temperature is increasing at a useful rate, or if it is dropping. For a more accurate summary of what is happening in the chamber, I rely on placing Orton cones around the chamber, and observing what happens to them through a spy hole in the kiln door.  The cones are made out of ceramic material, and are designed to flop over when temperature and time have done their work (you cannot reuse them!).  The cones started to go down sooner than I had expected, the ones near the top of the setting showing a temperature considerably hotter than the ones low down.  I had expected to fire to temperature in about 10 hours, but we did it in 8, and at the end I had to make a judgment call as to how much I should over fire the top few pots in order for the majority of the work in the kiln to be about right.  I certainly wanted the two large pots at the back of the kiln to reach a reasonable temperature.  Whilst this made things a bit tricky, the situation was not unexpected this being the first firing of the kiln after great changes had been made to it.

It was with a little trepidation that I opened the kiln.

The Kiln.... ,From Front to Back

At the front of the setting, as part of the bag wall that was built to deflect the flame, I placed some stoneware pots, and also put some unglazed earthenware wall planters to get well toasted by the flame and protect the glazed work. 

A planter for those with no more room in the garden..., you can hang this one on the wall!

I was delighted to find that the white clay that I use for crystalline glazed pots, became very like porcelain where the flame hit it.

The lowest level of pots in the front setting were all under fired, probably barely reaching bisque firing temperature. 

These ones all under fired, poor things!

I had directed the fire away from these pots too well!  I suspect they would have been about Orton cone 07-06 from their colour (Cone 07  984 C (1803 F),Cone 06  999 C (1830 F)). I will re-fire these pots.

The middle level of pots were about right, probably just short of cone 03 (Cone 03  1101 C (2014 F)).  

Various planters.

The ones on top were certainly hotter, especially where there had been a lot of flame contact, ranging from just above cone03 - to an estimated cone 01 (Cone 02  1120 C (2048 F),Cone 01  1137 C (2019 F)). 

Coffee pot and jug.  The coffee pot is waiting for its lid to be re fired.... 

Planter with black slip decoration.

I was delighted to find that the two large pots at the back of the kiln were both good.  

Big pot, about 25 inches tall (635mm)

Big pot with incised slip decoration, about 25 inches tall (635mm).

They could have had a little more heat, but were quite acceptable, I suspect they were averaging cone 04 (Cone 04  1060 C (1940 F)).

Things I will try to help even out the temperature in the kiln.

1) fire more slowly, this gives more time for radiant heat to even things out in the chamber

2) lower the bag wall so that less flame is deflected up and over the setting.

3) replace the bag wall bricks with old kiln shelves.  These should allow heat through, but keep direct flame away from work.

4) try firing with intervals of light reduction, this will make the flame roll around more in the chamber and stay there for more time (I was firing in oxidation for most of firing number one).

Nigella Stopit.

The Glazes...

The insides of any of the pots that could be used to hold drink or food were glazed with a lead free glaze.  It was one of those convenient white glazes for earthenware available from pottery suppliers as a powder, to which you just add water!  I "doctored" it slightly by adding 2 parts rutile and 1 part red iron oxide, so that it was no longer "refrigerator white", and also added about 5 percent ball clay to make it nicer to work with! 

The outside glaze was,

80 parts Lead Bisilicate

20 parts dry red clay the same as I used for the pot

10 parts Cornish Stone.

To this I either added nothing at all (for a honey coloured clear glaze), or, if I wanted something darker, I added 4 parts manganese dioxide.  On the few items that I wanted to be white I added 7 parts tin oxide to the base glaze.

Some pots had a white slip that I brushed over them whilst they were still freshly thrown, still on the potter's wheel in many cases.  This slip was probably, either 50/50 ball clay and china clay, or 90% ball clay and 10% borax frit.

One pot had a black slip under the glaze. After all this time I am not sure of what I did, but the slip was probably red earthenware clay, plus manganese, iron, and a dash of cobalt.

Just a further thought....

Two good reasons that I had for doing an earthenware firing for the first test:

1) an earthenware temperature of around 1100 Celsius (2014 F) would be much easier to reach than stoneware, and 

2) the colour that the earthenware clay fired to would give a good indication of where the hot and cool spots in the kiln were to be found.  This would supplement the information that the Cones and Pyrometer gave me about the firing.

Cones... are potter's sheep, they help you to sleep when you count them... zzzzzzzzz

For those who may still be reading this..... !!  Here are some Orton cone numbers and the temperature that they represent.  They are a bit tricky to use, because how quickly the kiln is coming up to temperature is an important factor too; the temperatures I have listed assume that the kiln was climbing at 150 C per hour (270 F).  The cones that I am writing about are the Large Orton cones, not the small ones or the self supporting!

Cone 07  984 C (1803 F)

Cone 06  999 C (1830 F)

Cone 05  1046 C (1915 F)

Cone 04  1060 C (1940 F)

Cone 03  1101 C (2014 F)

Cone 02  1120 C (2048 F)

Cone 01  1137 C (2019 F)

For those of you that really are reading this to help you go to sleep at night, you might also like to know that after reaching cone 01, Orton cones then are numbered 1,2,3,4,5,6,7,8,9,10,11,12,13,14...  zzzzzzzzzzzzzzz