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

2005-2006 Matthew Simpson

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ceramic: local clays: evaluation

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.

tile

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.