Wednesday, July 1, 2009

Using Microwaves to Dry Clay

Firstly, apologies for my lateness in producing information to the people that asked for it about getting started with wood fired kilns and about using microwave ovens for drying pots. As you will see from my last couple of posts, life has been very busy here, and time that I had hoped to find just did not come my way. I will begin with microwaves, and will write about kilns ASP. Sorry again for delays.


Some of you will have discovered that I have sometimes resorted to using a microwave oven for force drying pots.
This is a little How To description of what I have done, and what has worked for me. I should warn though that what I have done may not work in your case, may even be hazardous, and if anything goes really wrong, it may have the potential to void the guarantee that came with your shiny new microwave oven. So proceed carefully, if at all. You have been warned, so please don't drag me through the Courts!

Short History
A few years ago, I was commissioned to make several hundred ten inch floor tiles for someone that wanted to extend a kitchen. No doubt foolishly I accepted the challenge and set about making the tiles in the middle of our winter! Our studio reaches fridge like temperatures in winter that make drying work really difficult. As work progressed with the tiles, and every possible surface had a covering of them, the damp and cold became nightmarish, and drying ground to a halt. I took one of my damp tiles round to a friend one evening and shared my gloomy plight with him. His wife being conveniently absent at a choir practice, my friend said, "I know...., let's try the microwave!" I responded that said tile would either, a) explode, or b) stay warm and damp like vegetables do.
Half power and 4 minutes later, much to my surprise, the 2 kilogram 10 inch tile was presented to me still intact, steaming with enthusiasm, and noticeably drier. A short rest period and another 4 minutes in the microwave saw leather hard showing the beginnings of turning pale and gray. There was some degradation of the underside due to the tile sitting in a small puddle of water, but the result was so promising that I hastened to town and purchased a microwave oven the very next day.

What Has Worked for Me

I have only microwaved simple bowls, or tiles. The objects have all had a fairly even wall thickness, and have not had handles or other added parts.

Microwaving pots produces steam and puddles. It is important to use a microwave proof dish below the work to collect the drips, and useful to prop the work up on something. Failure to do so usually results in the bottom surface of the pot becoming wet and degraded with sitting in water. With the tiles I found it was good enough just to rest the tile on the raised rim of the dish (which was slightly smaller than the tile). I have also used small offcuts of bisqued tile to act as props.

It is possible to microwave pots that have air dried for 12 -24 hours. Really wet ones are problematic. They usually cannot cope with the huge shrinkage that happens when a pot goes from wet to dry and develop shrinkage cracks.

I never microwave to the point where the pot is completely dry. I take it to the beginnings of white dry. If taken further than this, the driest parts of the clay get too hot, and the work will explode with the steam pressure from within. I have also read somewhere that heating dry things in microwave ovens is bad for the oven.

With my microwave, 5 is half power and I find that 6 seems to be about the right setting to use.

As with ordinary cooking in a microwave, weight of what is to be heated is all important. Something heavy will take much longer to heat than something light.

Testing, Testing
I place my leather hard work in the microwave on little props in a microwave proof dish. I select half power, and microwave for one minute.

I observe carefully for signs of steam.

I remove the work and check for warmth.

I let the work rest for at least 2 minutes, check again for warmth.

I then run the test again on a similar size piece of work, (or on the original piece if it has been allowed to really cool down) this time at a setting of 6, which is just over half power with my microwave. If the clay has only felt about blood heat or less in the first test, I will try it at 2 minutes. If it has been more than blood heat, but less than steaming, I will try for about 1.5 minutes.

The idea is to carefully experiment until you achieve a power and time setting where the work will just get hot enough to steam gently when it is removed from the microwave, but not get so hot that it steams aggressively.

In the case of the 2 kg tiles, this was 4 minutes duration at power level 6. In the case of stoneware breakfast bowls, this was 1 minute only at power level 6.

After microwaving I place the work on a wire rack to cool for a minimum of 4 minutes, I then repeat the process. Usually it takes 3 to 4 times in the microwave to go from leather to just about to turn white dry.

When I microwave, I mostly have a production line going of several pieces so the microwave is in use all the time. It is a tedious process, but it is amazing how much can be achieved in the space of a morning.

It is important to keep an eye on the steam production each time the work is microwaved. If it starts to become aggressive, you need to shorten the duration of the time in the microwave, and/or the power level.

The above cautious approach has worked well for me. I have had few losses. I have put through well in excess of 500 items and have had, maybe, 3 or 4 pieces explode due to excessive steam build up. I have lost probably less than 10 due to rapid shrinkage causing drying cracks, but this has happened only when testing work that has been floppy hard rather than stiff leather hard.

Your Safety
Note that if at any time the work in the microwave starts to steam really aggressively, immediately switch the microwave off. Wait for several seconds before carefully opening the door. Allow the work to cool down until the steaming has quieted before removing the work from the microwave. Do not rush this, you risk having a face full of superheated steaming clay fragments if the thing explodes whilst you are peering closely at it.


Arkansas Patti said...

Aren't you clever but then so was your friend to think of it.
It is so neat that you share your findings. You really are a good teacher Peter. Anyone can follow your instructions.
However, your last paragraph is a bit threatening. Better thee than me.

Peter said...

Hi Patti,
What was the saying, "necessity is the mother of invention"? I think that applies in this case!

Regarding the last paragraph, I have never had a problem myself with the process, but part of that success is being methodical and always keeping in mind what might go wrong.

Jewels said...

Hello Peter! Thank you for the detailed instructions on the art of nuking greenware! It is interesting the water that is removed puddles at the bottom. I have copied your post to my pottery information file and am anxious to test it out (I will be sure to wear goggles). : )

I came across a post by Cold Springs Studio that might be of interest to you:

Hope you have completed your taxes and are licking the IRD envelope as I type.

All the best, Jewels.

Linda Starr said...

Hi Peter, what a thorough explanation, thanks so much, I have yet to try this as we are a dry climate but in winter I could use it for sure.

Peter said...

Hi Jewels and Linda,

I could do with a warm and dry climate at the moment, the nearest I can get to it is sitting in front of the electric heater surrounded by a pile of paperwork that describes the ebb and flow of our liquid assets! Ah well...

Thanks Jewels for the link to Cold Spring Studio, which looks most interesting and helpful. I will persue it all further after licking the IRD envelopes. I don't know if you have a similar thing in the States, but our IRD send envelopes that are too small for more than about one carefully inserted piece of paper, and the glue (which tastes absolutely foul....not even a hint of mint flavour!)is spread too thinly and miserably to work properly! We also have to affix a postage stamp. There was a time when posting stuff to the IRD was free! Our current government is actually cutting further jobs in the IRD, so..., I can only guess that the glue will be spread even thinner in the future!

Happy pot nuking! P.

Pat - Arkansas said...

A fascinating report of this process, Peter. I don't expect that I'll ever be in a position to do this, but should the occasion arise, I'll be prepared! :)

I greatly admire the kaleidoscopic image that heads this post.

Peter said...

Hello Pat,
You never know when such knowledge might prove useful! The image with this post was a photo of some pots on a drying rack that I fiddled with on the computer when I probably had something much more pressing to do! Fun anyway, and I thought its futuristic look probably fitted the theme of all that microwave radiation!

soubriquet said...

Back in 1986, I was having technical kiln control problems at my pottery, my father introduced me to a friend of his, a boffin who was retired, but still a part-time consultant to the electricity supply industry. Percy arranged the loan of some recording equipment to find out what was happening to my kiln. We became friends and were discussing accellerated pot-drying, Percy suggested a microwave chamber, they'd experimented with it for some other industry. I was a bit concerned about possibly reaching boiling point and bursting pots with steam pressure.
Percy's answer to that was a low power microwave. And infra-red remote sensors to monitor temperature rise, but the piece de resistance.... was that the whole thing took place in a vacuum chamber.... And so water vapourises and is sucked away. Of course, I already had a vacuum pump on the pugmill, so a bit of plumbing and a mysterious box later, we were vacuum drying.
However, the equipment could only take a few pots at a time, and the contraption was fairly bulky, so I abandoned the idea, and went back to my old shelving rack, wrapped in polythene, with a fan -heater ducted in.

That brought back a few memories!

Peter said...

Hello Soubriquet,
Thanks so much for your interesting comment regarding microwaves to dry pots, and welcome to my site. The vacuum chamber part of the process sounds a great idea. What a shame that some of these things are hard to scale up to a size where they would be useful for large numbers of pots! I think that the other solution to drying pots is to go to live in a country with a warm, dry climate!

soubriquet said...

The warm dry climate is appealing to me, after a cold wet week of northern english autumn, torrential rainstorm and flooding.
In my past, I've somehow spent time in very cold countries.
The first New Zealander I met was a guy I worked with in a pottery in Iceland! He was from Auckland, now living, potting, raising children, in Sweden.

Peter said...

A potter here, Peter Lange, made an ice kiln!! You can see a picture of it here
So, maybe the thought of a potter working in Iceland isn't quite so impossible as it sounds! I can't help but have visions of little updraft igloo kilns huffing and puffing their way to destruction at just above freezing point out there on the ice! Interesting that he should then move to Sweden (he must enjoy cold winters!

dni said...

Hi Folks, I'm an armchair potter enjoying to read your posts. Another way of heating up air is to build a stirdy frame to hold the pottery but covered in thick cardboard. Install two 100w light bulbs (the old fshion types) at the bottom third of the box, a door panel of timber and cardboard and a hole at the top and off you go with a cheaper Dehydrator. Can be used to produce jerky (biltong) or other food stuff as well

Peter said...

Hi Joesoap,
Good to hear from you. Welcome to my site, I am glad you are enjoying it. Armchair potting has a lot to recommend it! A useful idea of yours, especially as it can produce food as well! Kilns have been known to cook a good turkey or extra large chicken (although I haven't tried it!). Best Wishes, P