brick clay pile from a factory in Columbus Mississippi
Glaze Calculation
Clay Body Project 1
Problem: Formulate a working stoneware clay body
Objective: To begin to learn the basic components that comprise a clay body, to become familiar with the types of clay available for formulating a clay body, and to understand the importance of silica in a clay body and how it influences glaze “fit”.
Materials: Fire clay, Ball clay, silica, Feldspar.
Strategy:
Discuss the characteristics of a type of clay body you are interested in making for cone 10 application.
Consider the color you would like to have and the type of object you are making with your clay.
Formulate a clay body that includes ball clay, fire clay, and a silica component for glaze fit.
Add other components such as iron bearing clay or kaolin to influence color.
Instructions:
Discuss the formula you have chosen with your instructor
Mix a 500 gram batch of the formula in a sealable container by adding the dry materials and shaking or turning the container for 5 minutes.
Add water to the dry clay until the clay body becomes workable and plastic. (see water requirements for plasticity test)
Make test pieces to be fired according to the shrinkage test for drying instructions.
IMPORTANT:
- Correct measurement of materials is critical
-Completely mix all materials dry before adding water
- Focus on accurate measurement of test bars for determination of linear shrinkage
note: this clay body does not include silica or feldspar and would not work for making glaze ware.
j-peg by Jacob Thaden
Testing Clay Characteristics
Water Requirements for Plasticity
This test is used to find how much water is required to make any given clay body plastic. Clay bodies that require more water to become plastic are generally composed of finer particle sizes and prone to greater shrinkage rates. Clay bodies that require less water are generally composed of larger particle sizes and prone to lesser shrinkage rates.
1 Pulverize a completely dry sample of clay or thoroughly mix your measured formula and then pass the clay through a 30 mesh screen
2 Weigh out 500 grams of clay onto a non-absorbent surface
3 Fill a 500 cc graduated cylinder with water. Slowly add the water to the clay making sure it is well mixed after each addition.
4 Wedge the clay and add a small amount of water until the clay mass becomes plastic and able to model.
5 Record the amount of water used from the graduated cylinder
6 Apply the following formula to determine the percentage of water required to make the clay body plastic:
Weight of water used X 100
Weight of dry clay used
Shrinkage test for drying
This test determines the linear shrinkage of a clay body from the plastic to bone dry state.
1 Taking a well mixed and plastic clay sample make several bars of the following dimensions: 14cm long, 4 cm wide, and 1 cm thick.
2 On the face of each bar make a fine mark with “Ts” at each end that measures exactly 10 cm.
3 Allow the bars to dry slowly to avoid warping
4 Measure the mark on the tile from “T” to “T” after the bar is completely dry.
5 Apply the following formula to determine the amount of shrinkage from plastic to bone dry:
Plastic length – Dry length X 100
Dry Length
Shrinkage Test for Final Firing
This test determines the shrinkage rate of a given clay body fired to different temperatures. This test creates an understanding of the amount of shrinkage that can happen between temperature intervals.
1 Fire a test bar to a desired temperature (fire multiple bars of the same body at different temperatures for a better understanding)
2 Measure the length of the mark on the test bar
3 Apply the following formulas:
Total Linear shrinkage from plastic to selected temperature:
Plastic Length – Fired length X100
Plastic Length
Water Absorption Test for Fired Clay
1 Make test bars of a given clay body with the following dimensions: 5cm X 5cm X 10cm
2 Make at least 3 bars this size for each specific temperature to be tested
3 Fire the bars to the desired temperatures to be tested
4 Weigh each fired bar to the nearest centigram
5 Boil all final fired bars in water for two hours
6 Dry the surface of the bars with a clean towel and weigh them again to the nearest centigram
Apply the following formula to each piece to calculate the percentage of absorption:
Saturated weight – Dry weight X 100
Dry weight
shrinkage tests by Rex Halterman
