Richard Boyt, September 2003
After a number of attempts to find an ideal shape for testing the properties of a locally found clay, I have come upon one that looks promising. It is a disk 1 cm thick and 7.2 cm in diameter. This odd diameter was determined by a tin can I chose to use as a cookie-cutter. With it, I cut numerous circular disks from a 1 cm thick slab of moldably plastic clay rolled out with a rolling pin supported at its ends by two 24cm long by 1 cm thick metal supports. In this way, I can cut many uniform disks quickly and efficiently, each with a volume of 40.7 cc and a top and bottom surface area of 40.7 square cm.
To make the cookie cutter, I cut out both ends of the tin can. The can top was cut out in the usual manner, but the bottom was cut by slanting the can opener as it cut so that it also removed the thick ring of metal surrounding the bottom. The now-sharp bottom end of the can was then filed to remove any burr that would catch and hold the clay and so make the disk difficult to remove. A bottle slightly smaller in diameter than the tin can was used to push the clay disk down and out of the can. I marked each side of the disk with an identifying number and with six short, shallow, thin marks across its diameter. These marks were made 1 cm apart so that I could later determine the shrinkage of the disk as it dries. By weighing each disk as I measure it, I could figure the relationship between drying and shrinking. A graph of this shows a slightly curved downsloping line that indicates that the rate of shrinkage slows slightly as weight loss due to drying progresses.
Even though you turn the disks over frequently as they dry, it is difficult to keep them flat. Whichever surface is on the top tends to dry (and shrink) faster, and so the edges curl upwards. This is the reason for marking both sides and then averaging the shrinkage measurements as the disk dries.
I made the disks and dried them in sets of ten, so that I could test individual disks for various properties as I heated them to different temperatures in a kiln. This allows me to test different clay mixes for strength, plasticity, porosity, shrinkage, weight, thermal shock resistance, heat transfer, specific gravity, abrasion resistance, and maybe some other properties that I invite you to suggest.
I use a drop test to measure plasticity, a digital scale to measure weight, a metric ruler to measure length, a small electric kiln to fire the disks to various temperatures, scales to measure force required to break, submersion in water to measure porosity, a propane torch to measure thermal shock resistance and heat transfer, and a silicon carbide grindstone to measure abrasion resistance.
I invite ideas on how to better set up these and other tests that you think might prove useful. It all takes time, but should help make decisions as I add non-clay materials to the clay to obtain desirable properties that would prove useful in designing ceramic components for stoves. I've made a drawing that I can send to anyone interested.
Note: See other articles by Richard Boyt
- Ceramics for Stoves Part 1- Finding Clay
- Ceramics for Cookstoves 2: Testing Unfired (green) Clay Richard Boyt (May 2003)
- Ceramics for Cookstoves 3A: Test firing local clays- primitive kilns May 2003
- Ceramics for Cookstoves 3b: Test Firing Local Clays- Re-discovery of a "Natural" Kiln (June 2003)
- Ceramic for Stoves 4- Drying Formed Clay Shapes (September 2003)
- Ceramic for Stoves Part 5a- Making Samples for Testing
Practical Tips For Potters Making Improved Cooking Stoves
- Part 1-- Forward and Introduction
- Part 2-- Finding and Selecting the Clay
- Part 3-- Testing the Clay for Cooking Stoves
- Part 4 -- Materials That Can be Added to Make a Better Mixture
- Part 5 -- Making Up and Testing Mixtures, and Clay Preparation
- Part 6 -- Forming Stoves (June 2005)
- Part 7 -- Drying and Firing the Stove (July 2005)