this page created by Matthew W Simpson July 2005
all photos copyright Matthew Simpson please share information but do not take credit for my hard work
Clay is usually near water. Streams and rivers cut into the land, exposing the clay. A good indication of high clay contents of soils is cracking when dry. Beds and shores of rivers usually show evidence of clay deposits. Hillsides with their valleys, and everywhere that was once underwater millions of years ago should have a clay bed. The pure part of the claybed is called the vein. Major clay refiners mine their materials from these pure veins. Pure clay is nearly uniform in color.
You can test for clay content in a soil sample in the field.
Moisten the sample. Try to roll it into a coil (tube) with your hands. If it sticks together nicely, even when you bend it, the clay content is high.
This clay has fallen off the overhang and landed in the stream below. A mineral in the clay has oxidized and turned an orange-yellow on the outer surface. The unexposed clay is a uniform medium grey.
This is where you can observe the layering of the earth: coal, slate, clay and soil. All on top of a nice sandstone bedrock. This stratification occurs under our feet, but water erodes the land away to expose the layers. Just because you don't see the clay, or can't find it at the depth you dug, doesn't mean it is not there under your very nose.
Materials and equipment:
• shovels - round and sharp • gloves if you want
• old shoes, boots, waterproof is ideal
• buckets for hauling the dig
• notebook and pen for documenting the collection site
• ziplock baggies and marker for collecting smaller samples
• usual hiking gear (water, insect repellent, sunscreen, etc.)
The sample must be fully dry before refinement can begin. Spread your clay out on wooden boards, cement (or concrete), or plaster to dry. The left sample is wet, the right sample dry. A clay will change color from moist to dry stages.
Keep good records of your samples. If you find certain samples are well suited to your needs, you can later return and excavate more material. Keep a small sample labled in a reclosable plastic bag for comparison with all the stages in your refinement. You may also need to compare raw samples when relocating a deposit. These raw samples will give you a visual of the amount and kind of inpurities, sand concentrations, and since the plastic bag should be waterproof, a moist sample displaying the color of the clay as you found it, moist or not.
After drying, the sample must be crushed with a hammer, mallet, or otherwise blunt instrument. Be sure to wear eye protection, and possibly lung protection as well. When the clay is fine, sive through a coarse mesh, like quarter-inch hardware cloth. Discard any rocks and foreign matter, and pulverize the clay to an even finer powder. Sive this through a smaller wire mesh.
Slips are made by adding plenty of water to the powdered clay and mixing and remixing. It could take several days of hydration to create a silky smooth slip. The wet color of the clay is as vibrant as possible in this stage. It gives some indication to the fired clay when clear glossy glaze is applied. Be sure to keep records on your slips as well. The slips can be used to create a terra sigillata to color other clay bodies, but the "fit" needs to be tested or flaking can occur.
Materials and equipment:
• ruler • clay knife, pin tool, other clay tools
• rolling pin, or dowel rod for rolling tiles
• ware boards for drying tile samples
• notebook and pen for documenting the samples
• ziplock baggies and marker for collecting smaller samples
• electric kiln with kiln sitter and cones ^022 ^018 ^012 ^010 ^08
^06 ^04 ^02 ^2 ^4 ^6
Shrinkage test
take a test sample and roll out a 5" x 1" tile, 1/2 inch thick. With ruler and pin tool, mark the inch, half inch, 1/4", and 1/8" marks centered on the tile wet. Allow to dry. Fire the tile to maturity. Measure the marks and calculate the shrinkage ratio. It may be easier to calculate in metric. (4" = 102mm)
for example, if the measurement after firing was 86mm:
(86mm/102mm) = .843 then take 1 minus (.843) = 15.7% shrinkage
Maturity test
Roll out a dozen or so tiles, 10cm long, 3cm wide, using a ruler to ensure consistancy. Allow them all to slowly air dry. Fire them all to cone 022 or cone 018. When firing, place their ends across kiln posts so they have a chance to sag if they want.
Keep increasing the cone temperature in each firing, until the clay bloats, sags excessively, but don't overdo it. You don't need to melt the clay to prove it is overfired. You should come up with a temperature at which the clay becomes glass like, and will not absorb water. This is called being vitreous, and the clay is weather-proofed by reaching its full firing temperature.
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