Raw Material Testing
Aim:
1. To explore how the various chemicals and minerals used in glazes melt.
2. To examine the materials various raw characteristics.
3. To learn the names of each material.
4. To learn the health risks associated with raw minerals and chemicals.
5. Basic labelling rules.
6. To understand how pyrometric cones work.
Method:
1. Make test tiles that are small dishes, large enough to hold a teaspoon of material, these need to be bisque fired.
2. Each tile needs to be labelled with the material it will contain. If there is insufficient room on the tile then use an abbreviation. The mixture we use for labelling is a teaspoon of iron oxide, 1/2 teaspoon of manganese dioxide, a pinch each of clay, talc and flux all mixed together with enough water to make it to a consistency you can easily paint on.
3. Take one level teaspoon of each raw material and place in the appropriate test tile.
4. Fire the test tiles in your kiln to the same temperature as your normal glaze firing. Use the cone chart below to select the appropriate cone for your firing.
Analysis:
The following characteristics should be noted for each tile:
1. Type of clay used
2. Temperature fired to (use cones where possible)
3. Name of material
4. Characteristics of the raw material (feel, colour and weight)
5. Degree of melt (is it still powder or semi fused or fully melted)
6. Colour of fired sample
7. Opacity of fired sample
8. Reaction of surrounding bare clay (has the material coloured the surrounding clay)
Summary:
The purpose of this testing was to further your understanding of the various materials used, examining both their raw and fired properties. Cones are composed of the same raw ingredients as just tested. These glaze ingredients are formulated to melt after a certain amount of heat work. This means that it is not just how hot the kiln is, but also how long the firing took. The longer the firing the lower the temperature the cone will bend at (up to a point), see the table below. Cones are regraded as a more accurate indication of the temperature of the kiln than a pyrometre.
Orton Cones |
