Mat Slip Decoration

Obtaining satin-mat surfaces using slips

Have you ever seen satin-mat ceramic surfaces and wondered how it is done? Easy — by decorating with color slips made from the same clay body! Using slips made from the same body as your ceramic pieces is quite easy, but there are a few things to consider. The technique is outlned below.

The first quesion is how to make the base slip from your usual throwing or building body, i.e. the uncolored slip. If you are mixing up your clay body from dry ingredients, then you can skip the first part. A fine white earthenware, stoneware or porcelain is most suitable to this technique, although coarser clays can also be used. Dark clays are not really suitable unless you are thinking of aplying a lighter colored slip on top, in which case you will have to try out a different clay body.

Slip decorated mug by Elise Helland-HansenIf you are making up the slip from a wet clay, you need to roll out some of that clay into slabs and then dry them. Once dry, crush the slabs (the thinner they are the easier this will be to do), e.g. using a hammer or rolling pin or both. Sieve the powder through a medium fine mesh (wear a mask when doing this). This procedure will also remove any grog, if your clay was a coarse one. Next, add a small amount (like a drop or two) of sodium silicate to about a cup of warm water in a container. Slowly add slip powder to your water, stirring all the time. Some lumps may form, but don’t worry about this. Keep adding powder until you have a lquid roughly the consistancy of cream. Finally, mix the slip using a drill mixer attachment or other blender of some sort. Now you can strain the liquid through a fine mesh into it’s final container. The slip may thicken slightly on standing — this is due to the sodium silicate, which adds fluidity to the slip while reducung the amount of necessary water. So if the slip thickens in the strainer it’s not abnormal and you can help the flow through with a spoon or ladle.

Mat Slip Decoration
Obtaining satin-mat surfaces using slips

Coloring slips is similar to coloring engobes. Use the chart below to get an idea of what percentages of oxides to use to ge the desired effect. Stains may also be used, but you would have to experiment with amounts, or get the information from the manufacturer. Where available carbonates can be used in place of the oxides listed below. These are a bit finer than the oxides, but a slightly larger amount (try 10 %) is generally needed to get a similar color density. Ballmilling, especially if you are using oxides, will greatly help in reducing speckles and obtaining a uniform color density.

Colorant Color Amount (%)
Iron Oxide
Cobalt Oxide
Nickel Oxide
Manganese Dioxide
Black 3.0
Cobalt Oxide Dark Blue 1.5
Copper Oxide Mid Green 3.0
Yellow Ochre Ochre 4.5
Iron Oxide Mid Tan 3.0
Rutile Creamy Tan 6.0
Iron Chromate Dark Gray 3.0
Manganese Dioxide Purple Brown 6.0
Fire in Oxidation for the above color results.

Mat Slip Decoration
Obtaining satin-mat surfaces using slips

Slips should be applied to leatherhard, i.e. half dry ware. They can be applied by brush, dipped, sprayed or slip-trailed on. Because the same clay forms the base of the slip you are applying and we have added some deflocculant (sodium silicate), generally you should get a god fit. Any added oxides will help to flux the clay and possibly develop a low, satin-mat sheen. Using slips, a great many methods of decoration are possible, e.g. using wax resist or sgraffito (scratching back into the slip to reveal the uncolored clay beneath). An even veneer can also be applied to the whole pieces, giving the impression of a colored clay body.

Jason Green,  Watershed terra cotta, slip, glazeUsing the slip method of decorating, it is possible to once-fire your work, provided you don’t intend to use any glazes. Otherwise, bisque normally and proceed to add glazes as you wish! Firing can also be undertaken in any manner — in an electirc kiln, gas kiln, woodfired etc. Low temperature firing techniques such as pit firing or raku are probably less suitable, as the slip would not contain enough flux to melt suffiently. However, you could experiment with adding low temperature fluxes, such as lead frits.

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