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!
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.
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.
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.