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.

Work in progress

July already, where do the months go? It's close to the middle of winter and Dunedin is yet to see a decent frost. This would be the first year the lawns didn't stop growing, maybe due to the Chilean volcano or a nuclear winter, but it sure makes for some great potting weather! Here are some pics of work in progress.                  

Some miniatures and some scaled up pieces with lids. As they get bigger I may have to add  handles and lugs.

Yesterday I added a work bench and more robust shelving. This small room off the throwing area is where I wedge, store and recycle clay, place pots on the shelves to dry and have a bucket of creek water to wash up in.

Here are a few test glazes from the previous firing using sump oil as a fuel. This is Marcus O'Mahonys " Light orange slip" The recipe is:  Porcelain clay (dry weight) 20 ( I used the white stoneware clay body) China clay 10, and Tin oxide 3. All recipes on this post are found on pages 232 and 233 in Phil Rogers book:  "Salt Glazing".

Here is the same glaze but on a buff stoneware clay body (southstone) All tests were dipped at least twice to get variations in glaze thickness. Note the interesting pattern around the top of this example where the glaze was quite thick.

A very typical salt glaze effect. This mottled texture is called "orange peel" and consists of an extremely simple glaze of just two components: Ball clay 50 and China clay 50, fired to 1300+  with plenty of salt.

In this example of Arther and Carol Rossers' "Blue slip" consists of : Feldspar 15 (Indian ), Silica 8, BBR (clay ceram)?? 30.5 (I used Kingwhite kaolin), Eucalyptus ash 15.5 (washed pine ash), Titanium dioxide 0.5, Cobalt Carbonate 1.  Now that I look at this recipe it doesn't resemble the original much at all. Part of the joy of pottery is the unexpected.

Another A+C Rosser Glaze "Green slip to be sprayed over blue" However,  this example shows only the green slip - a very beautiful glaze on its own.  I still have the test pottle, so in the next firing I'll try it over the blue slip above.

 In my next firing,  I will be using bigger batches of glaze on larger pots and firing a slightly bigger kiln.  I'll be using canola oil from the fish and chip shop, and comparing this to sump oil -  hopefully getting similar results.  

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.