Monday, March 26, 2018

An Excellent Firing of the Wood Fired Kiln

Firing Notes.

Tuesday: I loaded the wood fired kiln. It was a hot, and the day reached 27 degrees C at some point, which is not bad for Autumn! I loaded as much as I could first thing in the morning, then did the rest after 5 in the evening, finishing by torch light.

Smaug, the cat, by night!
 Wednesday: It poured with rain the next day, so I was glad to have got the pots safely into the kiln before the deluge arrived. We split firewood in the carport and did the last preparations to the kiln in readiness for the firing. I loaded up the electric kiln and started a firing of some bowls that could not fit in the wood fired kiln. The bowls had glazes that really did not need a reduction atmosphere to work well.

Bowl from the electric kiln.

 I lit a small fire last thing in the day to pre-warm the kiln, then went to bed.

Thursday: A bit of a wakeful night had me finally out of bed about half past four. It rained heavily at times in the night, and still was spattering against the windows as I got showered and dressed, had a good cup of tea, followed by some coffee for good measure, then put on some extra warm layers of clothing before flinging open the back door and walking bravely into the night.

It was black outside, and a stiff Southerly flung splatters of wet stuff into my face. My stupid smartphone app claimed that it was 9 degrees, but it felt like 4 or 5 at most ... I don't think that the weather man had bothered to venture outside!

I lit a fire at 5.26am, and settled into kiln watching, feeding a little fire in the ash pit with sticks and thin pieces of split wood. My feet were getting cold. I listened to trucks snoring past on the Main Road, their tyres making fat frying sounds on the wet asphalt. I like firing in the rain. I have a blue tarpaulin stretched out over ropes behind the kiln to give extra shelter from the weather. It feels like camping.

Soon the inside of the kiln was a snug 100 degrees, and there was a merry crackle and snap from the fire and the smell of wood smoke and warming brick.
Thanks to the fire box and ash pit doors, very little heat escaped. Brrr cold feet!

Laura joined me out at the kiln at 6.30am when the kiln temperature was showing 150 degrees and took over stoking for a couple of hours. I brought out hot drinks and some breakfast.

My turn again when the kiln was just over 300 degrees. It is an interesting part of wood firing that every firing feels different, the weather, the wood, the way that the pots have been packed in the kiln, all have an effect, there are so many variables! This firing felt more demanding than the previous one. More thought and care was required to make the temperature rise, but rise it did! 

At 9.35am Laura was back at the helm with the kiln nearing 600 degrees. An hour later, with the kiln at 750 I took over, and kept going until the end of the firing.

I put the kiln reduction from about 800 degrees, and kept the fire fairly smoky until 1050 was reached, then I established a rhythm of -
stoke - watch for chimney smoke, wait - - for - - it - - to - - clear - pause for temperature to rise - stoke...

By fine tuning the length of the pause, I could fire reducing to neutral atmosphere, or reducing to oxidizing.

A reducing atmosphere is always hungry for oxygen, expect flames from spy holes when inspection bricks are removed!

1200 was reached a whisker after 1pm. I slowed the rise of the kiln and managed to keep the kiln over 1200 and slowly advancing for the next 2 hours. My pyrometer reads about 60 degrees low, so 1200 indicated is a reminder that it is almost time to start checking cones.

A pack of cones, from cone 8 to 10. I love the trails of incandescent gas that loop and roll through the chamber of the kiln.

At 1.30pm cone 9 was well bent in the hot top part of the kiln, and cone 8 was bending in the middle of the kiln.

Cone 8 bending... can you see it? It takes practice to see the cones in a hot kiln and it is important to wear welding goggles or similar eye protection.

At 2.45pm cone 10 was down at the top of the kiln, and cone 10 was half down in the middle of the kiln.

3.15pm last stoke of the kiln. Cone 11 half down in the hot top. Whilst I was cone watching, someone had discovered that a kiln chimney and the kiln shed roof is nice and warm whilst the kiln is being fired.....

Smaug, enjoying warmth worthy of a dragon!

At 3.25pm ten hours after lighting the kiln, cone 11 was down at the top, cone 10 in the middle, and cone 9 in the lower part of the kiln.

3.45pm the kiln was all clammed up and the temperature falling quickly.

Happy Smaug!

Smaug was delighted by the kiln, and spent the whole afternoon lounging around on the warm iron roof.

It was a very good firing. I was delighted to have been able to keep the kiln above an indicated 1200 for 2 hours (about 1260 C corrected). I do think that the pots benefit from spending time at that temperature.


Sunday morning, a first look into the kiln.
I was able to open the kiln Sunday morning. First glance in the dim kiln showed pots that looked a bit brown and drab, but unloading revealed some rather splendid things had happened after all!

Fascinating copper blue over copper red! Strange and extraordinary... I must do more of this!

I am delighted with the shinos, copper reds, and a new combination I am trying of a "fake ash" glaze (Van Gilder Ash) over an iron red (DeBoos 154).

The saturated iron red was superb in this firing, the kiln must be cooling slowly enough when below 1000 degrees for iron crystals to grow in a lovely way. The runny ash type glaze makes exciting rivulets over it, that are green in colour.

Van Guilder Ash over Deboos Iron Red

Van Guilder Ash over Malcolm Shino

Regarding the "fake ash", unlike the news, there is nothing "fake" about this glaze, it does have 15 percent of unwashed wood ash in it, but it also has 31 percent Whiting (calcium carbonate) as well, so whiting outranks the ash to some extent! Whiting can give very ash-like effects in glazes, but is easier to prepare and use than wood ash. Using ash and whiting together like this can give a more consistent result than pure wood ash alone, but it is still lively and interesting!

I like to tuck a special pot on a small shelf over the fire box of the kiln. Anticipating some runs from this glaze I, fortunately, put some offcuts of insulating firebrick under the pot. 

The pot has a rutile glaze over a dark tenmoku glaze. This combination usually gives a complex blue colour, but this time it is almost black with glittering small crystals and pale feathery streaks. Wonderful!

A rather spectacular heavy bowl with copper red inside, and rutile over tenmoku outside.

*Van Gilder Ash (I have fired this successfully from Cone 8-10). Very runny but behaves quite well at cone 8 - 9. You will find this recipe in John Britt's excellent Cone 10 glaze book, "The Complete Guide to High-Fire Glazes".

Whiting     31
Wood Ash  15
Potash Feldspar   5
Dolomite     2.5
Silica     22.5
Ball Clay     24

Red Iron Oxide     0.75
Rutile     4
Cobalt Carbonate     0.5

*DeBoos 154 (I have fired this successfully from Cone 9-11). This recipe is in Glazes for Australian Potters by J. DeBoos.

Soda Feldspar     50
Silica     20
Bone Ash    10.5
Whiting     6.5
Ball Clay     7
Talc     6
Red Iron Oxide 11 (gives iron red. 3 parts gives olive green).

More photos of pots from this firing will appear over the next few days. As you will have seen, Smaug is enjoying life at the pottery. He took a few days to get over the loss of his twin brother, Brian, but he has his sense of humour back again now and is good company.


Peter's Dad said...

Absolutely superb Peter; Congrats!

gz said...

Oh,firing envy!! The joy and hard work of sharing a firing.
I'll send this post to John Christie,marvellous potter and woodfirer extrordinaire. Scottish Potters shared a woodfiring in his kiln last year.

Anna said...

oh yes very nice firing. Love that large jar. Cats are canny things finding the warm roof.

Linda Starr said...

love your black cat hanging around patiently for the firing which was spectacular; love the blue and red bowl, so beautiful

Peter said...

Hello Dad,
Many Thanks for the encouragement, it is exciting when things turn out so well!

Hello Gwynneth,
Good to hear from you. Thanks regarding John Christie, I really must have a look for his work on line, I think a Scottish potter who called in here a few years ago mentioned him to me too. It is nice that he was able to make space available in his kiln for others to have a chance to try wood firing.

Hi Anna,
I was very pleased to be able to do some successful "surgery" on the jar this afternoon, it had a lot of glaze runs that had flowed past the bottom of the pot and into some insulating fire brick offcuts that I had placed underneath it just in case! It was quite a relief to be able to grind all that away and not lose any of the jar in the process!

Hi Linda,
The cat is becoming a very good companion when I am working outside. He really appreciates stacking wood and firing kilns! The blue and red bowl has me quite impatient to get back into making more things that I can glaze so I can explore that glaze combination further. It is really interesting how the blue seems to float and break up with the red glaze rising through it.

Arkansas Patti said...

I had no idea the time, care and attention needed for wood firing. Great that Laura is there to help. The results were splendid.
Got a giggle out of the cat taking advantage.

Peter said...

Hi Patti,
Giggles gladly given! Smaug, the cat, is an entertaining addition to the family here, he is full of curiosity and fun. I was concerned at first when he got near the kiln chimney, as I know it gets very warm indeed and I wondered if his skinny tail might get burnt, but no damage done! Keeping an eye on him, while firing the kiln, certainly made the time pass quicker!

Rhonda said...

What beautiful work Peter, the colours of the glazes are exquisite. I am sure your very happy with the results. So interesting ,reading your blog about the process.

Anonymous said...

Ah, more wondrous wizardry from your night firing. Tales of a familiar called Smaug, smoky night fires and how you turn lumps of clay into "glittering small crystals" could indeed be evidence of wizardry!! Anyway we love the results and if you do pop up more photos it would be good if you could include approx dimensions - to help prospective buyers. Graham & Amanda

Peter said...

Good Evening Rhonda,
Thank you for your encouragement! Good to see you today too! :-)

Good Evening Graham and Amanda,
Lovely to hear from you. Wizardry and magic definitely at work in the fire breathing kiln monster assisted by the Smaug! It is amazing what goes on in the swampy hollow by Post Office Creek! More photos have been taken and will appear soon, complete with measurements!