The descriptions and application advice given below are only an initial guide, almost all the rules can be broken in ceramics, however it is best to understand your materials first and to experiment.
Usually consist of one or more clays made into a liquid, sometimes with the addition of feldspars, frits or other fluxes. These slips can be defloculated but because of their high clay content and the amount of water to make them into a liquid they will shrink the most. So apply them to work that is still damp or leatherhard. If applied too thickly or to work that is too dry they can crack and fall off. Once fired they will usually produce a matt surface similar to any unglazed surface, the one exception is terra sigillata, which can produce a dull shine.
Best use is on leather hard clay that you want to cover with a different coloured surface, with a view to adding colour or glazing with a coloured glaze later.
Use: on damp to leatherhard
Clay content: 60-100%
Flux content: 0-40%
Consist of a mixture of materials, with clay forming about half the total. Because of their lower clay content engobes can be applied in a wide range of ways. The great advantage is their ability to be applied on work that is dry or even bisqued; it is also possible to apply them to leatherhard work. You will have to experiment to find the best method of application and thickness. Once fired they can produce a range of surfaces, usually matt to dull shine. Engobes form the base to underglazes, so adding colour to them is recommended, these will need a clear glaze over the top to develop their full colour.
Use: on leatherhard, dry and bisque
Clay content: 40-60%
Flux content: 10-50%
Consist of a mixture of materials, with clay playing only a minor role. Because of this glazes are best applied to bisque, although in some cases can be applied to dry work and then the piece can be fired just once. Glazes have the greatest variation in surface and colour; remember that the clay surface underneath the glaze will have a considerable impact on the colour and surface of the glaze.
Use: on dry and bisque
Clay content: 0-30%
Flux content: 30-80%
Below is a chart that refines the proportions of ingredients used at various temperatures to make an engobe
|Soda Frit or Feldspar||20|
If you have clays with a high shrinkage then you will need to modify the amounts of clay used. One option is the calcine (low bisque fire) half the clay content and then grind it up, this works best if the clay is already in powdered form.
If you are low firing and have no access to a borax frit then crushed clear glass will work as well.
The talc is added partly as a filler at low temperatures, partly as a flux at high temperatures and partly as it imparts a smooth creamy quality to the engobe in the wet form.
The zirconium silicate is an opacifier and help to whiten the slip.
To mix the borax in, first dissolve it in some hot water, then add to the engobe. It is used to make the engobe dry hard on the pot prior to firing.
Below is a list of colourants that can be added and the range of percentages used.
|Black||Black Iron Oxide||8%|
|Blue||Cobalt Oxide||2.5% - light < 7.5% - dark|
|Brown with specks||Manganese Dioxide||3% - light < 8.5% - dark|
|Brown||Red Iron Oxide||8%|
|Buff with specks||Ilmenite||6%|
|Grey with brown hue||Copper Oxide||3.10%|
|Forest Green||Chrome Oxide||2.5% - light < 7.5% - dark|
|Green||Potassium Carbonate||3% - light < 6% - dark|
|Pink||Red Iron Oxide||2%|
|Tan||Rutile||3% - light < 9% - dark|