Understanding Slips, Stains, Engobes, and Commercial Underglazes.
Slips
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. When put on wet clay you can make thick, textural effects.
Use: on damp to leatherhard
Clay content: 60-100%
Flux content: 0-40%
Fillers: 0%
Advantages – really gets into and stays on wet clay. Great for “combing through” or carving into.
Disadvantages: If your piece is on the hard side of leather, rather than the soft, they may crack off.
Tips: screen your slip, lumps show! Apply stains or oxide washes over slip at bone dry or after bisque.
Engobes
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%
Fillers: 10-50%
Fillers: 0-40%
COLOUR
COLOURANT
PERCENTAGE RANGE
Black
Black Iron Oxide
8%
Manganese Dioxide
3%
Cobalt Oxide
1%
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%
Buff
Yellow Ochre
2%
Cream
Antimony
5%
Grey with brown hue
Copper Oxide
3.10%
Cobalt Oxide
0.90%
Grey
Iron Chromate
6%
Forest Green
Chrome Oxide
2.5% - light < 7.5% - dark
Green
Potassium Carbonate
3% - light < 6% - dark
Olive
Copper Oxide
2.50%
Pink
Red Iron Oxide
2%
Rust-tan
Yellow Ochre
5.50%
Tan
Nickel Oxide
6%
Tan
Rutile
3% - light < 9% - dark
Stain
A stain is just colored pigment, commercially produced. On wet work you can just add water to a stain and
put it directly onto your clay. You can also color clays by mixing stains into the clay.
Once the work is leather hard, you’ll want to add a flux to the stain to make it adhere. For low or midrange 2-1 gerstley borate to stain will work (that’s our majolica stain recipe) for cone 10 try 1:1 or
substitute Frit 3195 for the gerstley borate.
http://www.bigceramicstore.com/info/ceramics/using-stains.html
Oxide washes
These are made with natural occurring minerals such as chrome, manganese, iron, cobalt, offer a more
limited palette, but more variations (speckles, fuming). They can be used directly on wet clay when mixed
with water, but once your clay is leather hard follow this recipe.
Commercial Underglazes
Essentially these are commercially produces Engobes.
Glazes
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%
Terra Sigilatta
A fine grained slip that can be burnished by a hand or chamois. Typically they contain a deflocculant
(Sodium Silicate, TSP) and are ball milled or siphoned so that only the finest particles are used. Excellent
in pit fire. Some terra sigilattas are also great high fire satin glazes – see the black one below.
Best applied to bone dry. May be applied thinly to bisque ware.
Making a sigilatta
Method A: Add dry ingredients to water in a glass or clear plastic jar. Shake vigorously for two minutes.
Set aside for 24-48 hours. Siphon off the top water layer. Siphon off middle layer (this is your sig). Leave
the layer of heavy particles; it can be used as a slip on wet ware. I prefer the simplicity of a turkey baster
for siphoning. When the clay is too thick to be sucked into the baster, you have arrived at the coarse
frained layer.
Method B: Add dry ingredients to water in the porcelain ball mill jar, have stones or balls take up 25% of
the jar’s volume. Grind for 6-8 hours; then pour slip through a sieve to catch the balls. Let settle
overnight. Siphon as above.
Recipes for gallon container.
Warm Black
14 cups water
Cool Black
Same as warm black but substitute a black mason stain for Mn and Cobalt.
6 grams TSP
1200 grams Red Art Clay
80 grams Manganese Dioxide
160 grams Cobalt Oxide
Non Peeling White
Very White (sometimes peels)
14 cups water
14 cups water
6 grams TSP
6 grams TSP
1500 grams ball clay
750 grams ball clay, 750 grams EPK
Orange
Turquoise
14 cups water
To white base, add Mason Stain 6266
6 grams TSP
One cup base to one Tbs. stain
1500 grams Navajo Wheel
or other fine grained red clay
Wine
Red Orange
To white base, add 1 Tbs.
14 cups water
Crocus Martis
6 grams TSP
1500 Red Art Clay
Brown Skin
Max Factor
14 cups water
14 grams water
6 grams TSP
6 grams TSP
1500 grams Black Mountain or
1500 grams Jordan Clay
other fine brown clay
Chrome Green (wear gloves)
Add 1 Tbs chrome oxide to 1 cup white base – looks like malachite