Clay Testing for Pottery Stoves

from a paper by Robert Hausner.

There is a wide variety of clays but a lack of specific information about them, and it is necessary to use simple, quick tests to determine whether they would be useful for making stoves. In Nepal three simple tests were devised by Robert Hausner for the FAO Community Forestry Development Project.

PARTICLE SIZE

Clays used for simple pottery items are rarely pure clay and also include silt and coarse particles. A mix of particle size is essential in making 'tough' stoves that can withstand thermal and mechanical shock.

Test Method

100 gms of dry clay are dissolved in 400 ml of water in a measuring glass and left to settle for 24 hours. It is then possible to distinguish between the clay fraction on top, the silt layer in the middle, and coarse particles on the bottom.

SPEED OF SEDIMENTATION

This is another way to measure particle size. Coarse particles drop out within 30 seconds, silt particles drop out more slowly, and clay particles can stay in suspension for up to 18 hours. A test can give a quick 'portrait' of a sample which can be compared to other samples.

Test Method

100 gms of dry clay are dissolved in 400 ml of water by rocking the cylinder with top covered for 5 minutes. The cylinder is then ''placed do and the settling is measured for 6 minutes.

RELATIVE WATER REQUIREMENT

The amount of water a clay sample can absorb is related to the amount of water the different types of clay particles absorb, and the amounts of those clay particles in the total. The more water a sample absorbs the more it will shrink during drying.

Test Method

100 gms of dry clay are put in a glass and water is slowly added and stirred in. The water requirement is reached when a line drawn into the mix with a pointed stick does not keep a groove line shape anymore.

PLASTICITY

Although plasticity is an important characteristic for pottery clay its presence does not necessarily imply that the clay is of good quality for stoves because it might only be the effect of organic impurities and acidity which, while making clays with a high silt content workable on the potter's wheel, results in a poor fired-strength after the organic 'glue' is burnt out in the kiln. A plasticity test cannot be depended on to give consistently reliable results.

While the specific results of the tests do not match in all cases the best mixes have the following characteristics

- high amount of coarse materials
- as little fine sand and silt as possible
- at least 30% clay by volume in a 24 hour test.

The graph of sedimentation rates shows a close similarity between the clays used to make improved pottery stoves in four countries. The bar chart of water requirement shows similarity for the water requirement of good quality stove clays with the exception of (and the reason is not clear) the Sri Lanka sample.