Pros and cons of fuels

Over Easter I fired fibre and brick kilns, with varying degrees of success.

The gas kiln is always easy, being gas, but brick and fuel kilns are a different matter. After about two years of experimenting with different fuels, I have decided to go back to diesel for the following reasons...sure, sump oil from a local garage is a free fuel source, its viscosity is also good, being mixed oils and transmission fluids. All of these burn extremely well and it gives a certain sheen to a glaze like only oil can, but it would have to be one of the most filthiest substances known to man. It's also quite possibly carcinogenic and turns the potter pitch black. After working with how clean gas is, I would prefer it to be like that for the brick kiln.

Which brings me to... vegetable oil mixes; the best one being a 50-50 mix of diesel plus canola oil from the local fish and chip shop. This also burns good and gives nice results,  but it has to be mixed and for best results put through a filter so as not to clog up (even slightly) the taps and burners. This can be time consuming, unless one enjoys this sort of thing.

The final straw for me was when it came to what was taking place in the fuel tank. I thought viscosity was the problem, then I saw the crap that came out of the tank! I had put clean canola oil in and it grew some sort of green algae slimy shite and a lot of it - that would slow down the flow somewhat. The sump oil had also left behind metallic dregs like small flakes and even little twists of metal, that looked like little snails, all packing down into a solid mass at the bottom of the tank. Sometimes being frugal doesn't always pay off,  but you have to try these things to see what does.

Preheating fire box .In this firing, I came in with desiel at about 300 degrees centergrade.

Once the diesel took hold I could control the rate of firing, just like the gas kiln, going from 300 C to 1300 C in around 8 hours.

Lunchtime in the best of company.

Silvery sunset.

Watching cone 8 go over, about to throw the salt in for one hour, just after sunset. As for atmosphere, it was going to be oxidised but with opening and closing the dampner and winding the fuel up till it looked like a dragon breathing through it (reduced),  or knocking the fuel back to slow it down a bit, creating a crystal-clear (oxidised) atmosphere inside, who knows what the outcome will be?

Interesting results and heaps of beautiful glaze tests to analyze.

Some of the better pots from the kiln, alongside cones 8, 9 & 10.

Diesel simplified things so much, no clogging of taps and burners, no oil slick all over my face, arms and legs, and the results were awesome. It's as close to using gas as I can get without using gas. It's worth the extra outlay to be problem-free.

Rough and ready

A quick post on salt firing...

In this photo I have cut a bottom shelf  and placed it on some props.I have also installed a crude bagwall.This time around I hope to not fire too much over 1300 deg c. because it plays havoc with my  kiln shelves making them hard as. If anyone out there has experienced cutting silicon carbide with a masonry blade you will know what I mean...The heat...the dust and these shelves aren't light either.

Tomorrow I will cut a third shelf and work on the chimney, kiln wash the shelves, etc and on next day - glazing and packing. Hopefully I'll be ready to fire up on the weekend, although I was planning a trip down the harbour to gather some cockle shells (for that nice wadding effect that everyone does) and a trip to a local forest for some pine needles for my raku firings. Good thing I'm not in any kind of a hurry... rough enough.

Winter Studio

It's been a busy month and the weather is unusually warm for this time of year, but now it is time to move down to the winter studio before the cold weather closes in. I have been working on fixing up an old shack that I have been using as a studio for a number of years. It is actually an old pioneer cottage that was built in the 1860's. It's seen better days but that just adds to the character and most importantly, it has an open fire.

Autumn colours of wilding cherry trees at the gate.

The potter's cottage and salt kilns

The old kitchen, transformed into a throwing area. Test pots thrown off the hump, drying, and ready to be dipped in glaze and raw fired.

The old bedroom window. I have been using plastic to keep out the elements and have been picking up cheap windows where I can. So far I have replaced four windows. It's so nice to have a view through glass and it's much warmer than plastic!

The old parlour (or sitting room),which I've furnished from the local refuse recycling shop at Green Island. A good place to relax and warm up by the fire.

These are the salt glaze slips that appear on pages 232-233 of Phil Roger's excellent book, 'Salt Glazing' (2002). Applied to raw test pots before going into the kiln.

Cutting shelves to size, and using some of the off-cuts as props. I also cut up old fire bricks for larger props. These are all then shelf-washed to protect them from the salt and hopefully make them last a little longer.

Here I am checking to see if the stack will fit in the kiln. I had about a centimetre to spare!

Steam created when throwing salt into the kiln. I started salting when Cone 9 went over and threw in 14 pounds of salt over the duration of one hour, which melted Cone 10 (1300 deg C) in the top and bottom of the kiln, so it must have reached Cone 11 (1320)  or 12 (1340-50 deg C, given that there's about 20 degrees between cones), but it made for some beautiful high-fired results.

The next morning I got home from work to open up the kiln and see the results, which were very much to my satisfaction.

In this firing, I used sump oil as a fuel and burned it cleanly in an oxidising atmosphere, attempting to achieve copper greens and other various glazes, which I succeeded in doing. Out of about 20 glazes, I got five or six nice ones, which I can make bigger amounts of glaze and scale up the pieces.

Small Salt Glaze Test Kiln Construction

Last week I built myself a small makeshift top-loading brick stoneware kiln, that would give me the opportunity to test out salt glaze recipes in small amounts. It is fired initially by wood and then by a diesel jet burner going in at the side. I was able to put it together in one afternoon, using a scaled-down layout from the one that I used on my big kiln. This one has a kiln shelf as a roof and no arches.The walls are a single layer of fire bricks, as this cuts down on a lot of bricks, and should be enough for a small test kiln.  Anything bigger needs to be double brick walled.

It's the same size as my small gas kiln and would make a great hobby kiln/test kiln. I'll be interested to compare it to the small gas kiln in terms of fuel economy. I've used fire bricks throughout (house bricks will just melt at these temperatures) laid on a bed of cinder blocks. The kiln reaches temperatures of over 1300 degrees Celsius, so it is suitable for any kind of stoneware.

Here are some photos showing four stages of the construction:

The first layer of fire bricks, sitting on a foundation of cinder blocks

The second level, showing the firebox and throat into the kiln which runs under the floor.

Side view. This is where the diesel burner goes in, under the chimney. It goes under the floor combustion space before it gets into the kiln.

Top view, showing kiln chamber at the front, chimney, and firebox at the rear. There is a salt port in the top right hand corner, where the salt is thrown in over the flame, in front of the bag wall. This photo shows three layers of bricks in the main chamber where the pots will sit. After this I kept adding layers till I got the finished height, adding one spy-hole in the middle, and a little hole for the pyrometer up near the top. There will be a flue for the chimney at the rear.

Side view showing kiln shelf covers in place, and chimney - I will add a stainless steel flue chimney as well, before firing, to create more draw.

I hope to fire this one in another week or so, with some new salt glazes and subsequent oxidisation and reduction firings.

There are some plans similar to what I used here. They were originally taken from 'New Zealand Potter' magazine but unfortunately I forgot to write down a reference.  Here is a plan showing a cutaway view of the kiln in action: