Thursday, December 10, 2020

Wood fired Planters

The Wood Fired Kiln with planters.

Earthenware planters.

I am not sure if my brain really does work at 4 in the morning, I made a start at a new blog post then, but it is now 4.30am and this page is still blank*. I have been writing with the goal of describing the happy pleasure of firing the wood fired kiln on a rainy day, but keep not finding the words. 

I do keep a record of kiln firings on graph paper, with short, rather terse, notes such as "Cloudy and still day around 10 degrees. Chimney damper fully open", or "Raining", or "Still raining!" And in the last hour or two of the firing there are short hand coded utterances such as "T^01r" (which is to say that Orton cone number 01, located in the top part of the chamber of the kiln, is bent over about half way).

But these jottings and charts really do not communicate the experience of firing the kiln, which is really not a "wordy" experience at all, but one of the senses.

We fired the wood fired kiln on the last Monday in November, an activity that was for the most part accompanied by the splash, plop, rattle and hiss of heavy - rather weepy- tokens of rain that bounced and exploded into a shrapnel of misty small drops whenever they hit the corrugated iron roof over the kiln and the nearby wood shed. 

A temporary tarpaulin had been rigged up to extend the "dry" area behind the kiln, but in spite of a patchwork of black sticky tape, there were gaps and numerous small holes through which water could ooze and drip. We sat on collapsible camp chairs, Laura on one, with a thick newspaper on her knees for insulation and waterproofing, and I with Mr Smaug the kiln cat on my lap, and a steepled roof of newspaper over his back to protect him from a persistent splash of water from a hole that was in the tarpaulin above him. 

The kiln was lit at 6am, and the fire was soon happily popping and chuckling away without much smoke or fuss, because the kiln was already warm and dry inside, thanks to a small fire I had maintained in the ash pit for about 4 hours the day before.

The kiln loaded with earthenware planters prior to bricking up the doorway.

Packed in the chamber of the kiln were earthenware planters, these were without glaze so could be stacked rim to rim and food to foot. I used no kiln shelves at all in this firing, a first for me, and of course I feared the towers of pots collapsing, or the planters welding themselves together. I had used wadding to separate bases of pots, from each other and to give a level base to stand on in the kiln. I did not want to use the regular alumina, china clay and grog mix that I would use in a stoneware firing, as this leaves white deposits on the clay that doesn't look good on red earthenware. Instead I made wadding from earthenware clay mixed with a generous quantity of sand and some grog. I kept thinking about what a "traditional country potter" would have used for such things, and I am sure that they would have not rushed to the pottery supply to purchase alumina hydrate and other such fancy stuff! I hoped the earthenware wadding would work. I had also made some significant modifications to the kiln inside... so these added up to quite a long list of "unknowns" to have lurking in the recesses of the mind.

The first 5 hours of the firing were taken slowly at an average of 70 degrees per hour until we reached 350 (158 F/hr until 662 F). The pots in the kiln were all raw, and being planters some were quite heavy so any moisture had to be got rid of with care. From 350 to 650 (662 - 1202F) the pace was increased to 100 degrees per hour (212F/hr). Once a good red heat through the whole chamber could be seen, the rate was accelerated further, and we climbed at a steady 140 degrees per hour (284F/hr), and then slowed this for the final hour of the firing to try to even out the heat in the chamber which was noticeably hotter in the top.

Graph of the firing. Temperature is in Celsius.

I was delighted with how controllable the kiln was throughout the course of the 11 hour firing. I did not need to rake the firebox at all, and maintaining a very steady increase in temperature was easy to do. The only difficulty was with the top of the chamber being hotter than the bottom, it made the question of when to finish the firing more of a challenge, but I had set a cone 4 at the top as an absolute "do not exceed", and was greatly relieved to see cone 03 go down in the cooler part of the kiln, just when the drooping of cone 4 meant that it was imperative to call a halt. Given the rate of temperature increase of the kiln throughout the last 2 hours of the firing Cone 03 would be about 1095 Celsius and Cone 4 about 1178 (2183 F and 2152 F).

Fired pots.

The hottest pots were toward the back of the kiln, the ones at the front were cooler.

The kiln was opened Thursday morning. The first impression was that a lot of pots were over fired, but in fact the firing was much better than it first appeared, the toasty pots look gorgeous, and most of the larger pots had others inside them that were well protected from direct flame. 

Separating pots with a rubber mallet.

I had to use a rubber mallet and a few taps of a wide metal scraper to free some of the pots that were rim to rim. They had stuck quite firmly, but released from each other without damage. The good thing is that the wadding that I put between the bases of the pots, when they were stacked base to base, worked really well, and did not leave nasty marks other than occasional indentations when a pot had got so hot that it had softened enough for the wadding to push into the clay.

Removing wadding from the base of a pot.

I am currently making some stoneware bowls, tea bowls, jugs and other useful things that I hope to fire in the wood fired kiln before Christmas if at all possible. It will be very interesting to try the kiln at stoneware temperatures. The changes I made to the the kiln have increased the pull of the flame around the inside of the arch of the kiln, and has strengthened the flow of flame across the chamber. I think that ash glazes and shino glazes should really "sing" with this cross draft arrangement.

The checkered floor of the kiln chamber. On the left the heat enters the chamber via three inlet flues. After circulating in the chamber, gasses are exhausted through openings in the checkered floor and make their way to the chimney.

Another view inside the kiln. Lots of "fine tuning" of the flow of flame inside the chamber is possible, as I can vary the size of the inlet ports and can block, or partially obscure some of the outlets in the floor checker to help direct flame around the chamber.

Happy pots in the morning sun complete with pepper seedlings.

 Now almost a week has passed since I wrote this post, and it was rather a busy one, so getting this finished and photos added to it has taken a few days. I have potted up some peppers that I grew from seed and it is lovely to see a few of the planters with something in them. 

I'm hoping to get another post written in the next few days - before Christmas - !

*Why write a blog at 4 in the morning when the brain isn't working? A good question! We have had cats in residence here since the 1990s, and most have adapted themselves to our waking and sleeping habits very well, curling up comfortably behind us in bed, and arriving, stretching and yawning for breakfast at about the time that we would be making our first cup of tea. Nigella Stopit has been the exception to the rule... she currently thinks eating just before dawn is OK, and will do all she can to assist us to fit in with her desires and needs! She has a keen intelligence and an impressive vocabulary and vocal range!


Anna said...

Hi Peter so glad the firing went so well for you. those pots look great and will grown some great plants I'm sure. Merry Christmas to you and Laura

Peter said...

Hi Anna,

Lovely to hear from you. Amazing how rapidly this strange year has gone, and quite a shock to the system to hear carols playing again so soon after last time, but.... Merry Christmas from us to you! :-)

gz said...

Good to hear from you and see that lovely firing!
(Just make sure that the planters don't narrow in at the top...just try getting a potbound plant out of one that does!!)
We are truly missing our Aotearoa New Zealand friends sorely, we wish our second home was our first!!
Cold and wet here...ground like a sodden sponge...meh!!
But there are cards to finish, all the family is well and at least we are in contact with friends around the world .
Sending a big hug to you both and the cats xx

Peter said...

Hi Gwynneth,
Good to hear from you. We think of you and the Pirate quite often, especially when visiting the Karitane Store where we had wood fired pizza! It will have been a very difficult year for you being so isolated and unable to travel. I do hope that this coming year starts to get better again now that vaccine for Covid is starting to be rolled out.
Cold and wet where you are, and... parts of the South of NZ were very cold today with snow to 500 metres and a feel of winter on the wind. Yesterday was the complete opposite, with real heat in the sun and a blue sky overhead.
Mmmm, you are right of course about narrow topped planters.... I just can't help myself sometimes! Mostly I am fairly good about such things though.
Thanks for the hugs. Big hugs heading North to you and the Pirate!

Linda Starr said...

amazing to see how you stack them, hope you get lots of peppers

Peter said...

Hi Linda,
Lovely to hear from you. All sunny and warm today so the peppers might think that life is good and there is growing to be done! I really don't know if I will have much luck with them as I have not done all that well with tomatoes in the past, let alone peppers, but I will do my best to keep them cheerful!

Stacking pots in the kiln the way for the first time felt quite daring, but it is certainly how many potters worked in the past. I'm delighted that it worked so well as it makes it much nicer not having to fit work around props and kiln shelves.

charlie said...

Hi Peter. Was wondering how the firing went. Very successful I see. Very well called with the delicate balance between too hot at the top and underfired at the bottom..Cheers Charlie

Peter said...

Hi Charlie,
Thank you for that.. I am glad I put a cone 4 - never exceed - in where I thought it would get a bit hot. As it turns out I actually missed the hottest part of the kiln, which was at the back, so some pots were definitely near their limits, but the earthenware was remarkably forgiving! :-)

charlie said...

Any idea on why it hotter at the back Peter?.It is a nice looking kiln and I like the way you can change the flue and checker floor. We have had a lot of seconds. Thirds actually with s cracking in the bottoms of mugs. Mainly due to the softness of the clay and dodg

charlie said...

Ha went before I finished. Dodgy clay recipe as the Aussie clay we also use isn't playing up the same. Getting some sales from home now

Peter said...

Hi Charlie,
How frustrating for you having clay problems and lots of seconds... very difficult when clay changes and reveals its wicked ways!

The uneven heating of the kiln was expected, but I needed to fire the first firing with this changed kiln interior with all flues being equal size, so that I could see more clearly what needed to be changed. The good thing is that unglazed terracotta shows heat variation so well, so it was very clear as to where the hot and cool places were. My feeling is that the flues into the chamber need to graduate in size (smaller to the back), and the exits through the checker floor could be restricted towards the back of the kiln so as to encourage the heat to come forward. It will be fun playing!

charlie said...

Scary fun haha.I am in the process of trying to decrease the expansion rates of my glazes as I had a complaint with a vase leaking. MATRIX is very handy but I have just made up some test tiles for the real tests