Slip, Slap, Slop. By Janet Kovesi Watt

Notes on slip decoration

(Preliminary clarifying note: people sometimes confuse the slip used in pottery decoration with the specially modified slip used in slip-casting, the more so because plaster moulds are often used when making slip-decorated dishes. These are made, however, by press-moulding, with the clay treated like pastry for a pie, and using the clay as clay.)

The term “slipware” generally refers to slip-decorated earthenware, but slips can be, and are, used at any temperature and on any clay. The use of slip to decorate pottery has a long western tradition in which we can feel at home, and which we can try to explore in a fresh and personal way. We are unlikely to achieve the same charm as the 17th and 18th century Staffordshire or Pennsylvania Dutch potters, still less the truly virtuoso artistic achievement of Thomas Toft (though English potter Mary Wondrausch comes close), but the technique offers many opportunities for a contemporary, even sophisticated expression, and one can hit on combinations of slips and glazes which give unforeseen and possibly spectacular results. The techniques, however, are fundamentally simple, and have been used by peasant potters for centuries. As Michael Casson says: “Slip, once understood, is really the kindest of masters and will change the colour, texture and even visual proportion and balance of your work for you.”
Michael Cardew said he loved the way that a pot seemed reborn when covered with gleaming fresh slip.

Slip is just liquid clay; there is nothing mysterious about it. People often ask for a recipe, but I have found, as Leach says, that slips adhere best if they are as close as possible to the body clay in composition. For a dark slip you only need to add the required percentage of colouring oxides to the body clay itself, and pale colours are no problem either if your body clay is itself pale.

(When I first started to make pots in WA in the mid-60s, the electric kilns of the time were limited to earthenware temperatures. The clay we used was the brown-firing clay supplied by the drainpipe factory on Great Eastern Highway in Belmont, on the site of the present “Ascot Waters” development. It was a plastic, strong clay, but seemed to undergo a final shrinkage just before becoming bone dry, and slips made from pale clays had a maddening tendency to flake off at the last moment).

Making up slip.
Assuming that you are using a standard buff-cloured stoneware clay, save up your scraps and trimmings and let them become bone dry. You can then weigh out a batch, and add an accurate percentage of colouring oxides. For a paler slip base, use a plastic white clay like Ball clay FX, perhaps combined with a proportion of body clay. I also tend to add 10% of feldspar, and will in future also add 5% of whiting, to act as a flux and help the slip to adhere to the body. Les Blakebrough years ago recommended using Walkers no.10 as a pale slip, and of course the other white-firing bodies now available will probably work well. I do not recommend using BBR/ClayCeram in a slip, as I have found that it has a wetting effect on the pot to which it is applied, and handles are at risk. Add the dry ingredients to water, and allow them to slake before sieving through 80 mesh, which I find quite fine enough. If you find that your slip is insufficiently plastic to fit the body, you can try adding a small proportion of bentonite, say 3%. Remember that when mixed with water it behaves like flour when mixed with hot water, i.e. it swells and goes lumpy, and needs to be very thoroughly mixed with the dry ingredients and allowed to slake completely before sieving, otherwise the lumps stubbornly remain, even with repeated re-sieving.
Colouring slips.
Generally a slip needs twice as much oxide as a glaze.
Daniel Rhodes in “Clay and Glazes for the Potter” has useful suggestions for additions of colouring oxides, as does Michael Casson in “The Craft of the Potter”.
Tan: I use 5% red iron oxide, plus 6% of whiting as a flux, as I have found that at stoneware temperatures the iron slip had a scorched, metallic appearance.
Black: I used to use: manganese dioxide 6%, red iron oxide 1%, cobalt oxide 1%, but now use 6% of a commercial stain, again with the addition of 6% whiting.
Blue: cobalt oxide or carbonate 1/2% - 1 1/2%. The carbonate is slightly weaker in colouring power, and more identifiable (mauve) in the raw state.
Cobalt is such a strong colourant that it is worth making Katherine Pleydell-Bouverie's “cobalt mix”.Grind up one part of cobalt very thoroughly in a mortar with three parts of silica, and then add three parts of feldspar and three of kaolin and mix all together, so that there are ten parts in total. You will eliminate any specks of harsh blue, and can use the diluted mix in decent-sized and therefore accurate quantities, since 5% of mix represents point five of actual cobalt. It is a laborious procedure however. If you have access to a ball mill the ingredients can be mixed with water and set rolling, the fine slip being dried out on plaster afterwards. Alternatively, use a commercial stain; 5% would be a likely quantity to start with. Make line blend tests to get precisely the shade you want.
Green: In oxidation copper oxide (black) or carbonate (green) 4%-5%
In reduction: chromium oxide 3%, plus perhaps cobalt mix 1%.
Brown: manganese dioxide 6%-8%. Curiously, as Michael Calder discovered, 4% of manganese in a clear glaze containing both whiting and talc gives a pleasant muted gold colour.

These are just suggestions, and everyone develops their own mixtures and combinations. Rutile gives interesting yellowish tan colours, and is worth combining with other oxides, and there are a number of commercial stains. The yellow stains do not survive a reduction fire.

Applying slips.
Success depends on a) the consistency of the slip, and b) the condition of the pot.

a) A pouring slip needs to be of what cookery books call a “coating consistency”. (Before milk was homogenised one could talk of a slip being slightly more liquid than top-of-the milk.) It needs to be thin enough so that when poured or dipped onto a pot it flows cleanly over it, without drooping back on itself like badly-applied paint. Try it out by dipping a finger – you should be able to see the outline of your fingernail under the slip coating. Pouring slips are likely have enough tolerance to stick satisfactorily.
Slip for trailing needs to be thicker, but not so thick that it stands up in little peaks when stirred. Aim for little blobs rather than peaks. It should barely drip reluctantly from the tip of the trailer. It needs to have a liquid, flowing quality so that it does not look like cake icing. Leach gives the useful tip to add vinegar to a trailing slip, particularly if it turns out to be rather more runny than you wanted. The vinegar acts as a flocculent, and stiffens a slip in an amazing way, while in fact diluting it so that it dries to a satisfying low relief. I have never had a slip so doctored lift off a pot.

b) The pot needs to be at the right stage of leather hardness – firm enough not to sag or soften with the wetting (make sure that handles are rigid) and damp enough to “marry” with the water in the slip, so that the clay particles are sufficiently lubricated to accommodate all this extra water. A pot which has started to change colour at the edges is at risk, though you may be able to slip it if you sponge down or lightly spray first.

Tools for trailing
Many people use the rubber bulb-type trailer, which needs to be kept under constant light pressure
so that it does not suck air in, since there is then the risk that it will blurt out an unsightly blodge of slip and ruin your design. Shake the slip down before trailing, and squeeze a tiny blob out first.
I always used to use a cake icing bag and small plain nozzle, which I found more relaxing to use, and easier to control, since it works on a gravity feed.
Mary Wondrausch devised a tool consisting of a section of bicycle inner tube, closed by a cork into which a 5cm length of a discarded ballpoint pen is inserted. The pen has its writing tip pulled off, and the fine (empty) tube for ink is threaded through instead, extending about 2cms. It is fixed in place with araldite. Its length slows down the flow of slip. The rubber inner tube is then sealed by a bulldog clip at the other end.
This does work very well, but is fiddly to fill, does not hold much, and is uncomfortable to hold, so I devised a variation in which the cork and ballpoint combination were fitted into a conical icing bag which is easy to fill, and can be twisted and turned over to seal.
Simpler still is Andrea Vinkovic's tool consisting of a ziplock bag with a tiny hole cut in the corner.

Trailing on a vertical surface
It helps if the background is moist, so that movements with the trailer are lubricated. If you want to trail straight onto the body, it is helpful to spray lightly or brush with water.
If trailing onto a background slip you need to wait until the first glistening wetness has dulled a little, or the design will slide off, but not so long that the surface is completely mat, or you find yourself trying to drape ropes of slip over the pot. The trailer needs to be held a minute distance away from the surface – too close and it will dig in; too far and the slip will fail to make contact and sag off. And all this has to be done on a curved surface. A banding wheel is essential.

Trailing on a flat surface
This is easier, since gravity is on your side, and decoration can be done before the clay is leather-hard, if it is supported. It is best to decorate a dish before it is turned, since it is thicker then, and can survive the wetting and softening. Moulded dishes are supported by the mould itself, so can be slipped immediately. Be careful to wipe off dribbles of slip overlapping the edge of the dish, as they can fasten that part of the dish to the mould and prevent it from being able to shrink, thus causing cracks.
Decorating a slab.
Leach describes the technique of first pouring a dark slip, and immediately trailing lines across as close together as possible, using slip as liquid as you can control. The slab is then bumped down to spread the trailed lines, which however should stay distinct, with often magically fine lines of dark slip between. While it is still wet the traditional feathered patterns can be made, using a pin, a stiff broom bristle or the stripped end of a feather.
Forming the slab into a dish is not always easy. It is best to shape it over a hump mould, so the drying must be watched. It needs to be non-smudgeable in the middle, but pliable enough round the edges to be coaxed into shape over the hump.
I use a pair of moulds – first a press-in mould, into which I lay a slab of clay as though to make a dish. I pour plaster into that to form the hump, and when it is dry I cover it tightly with fine knit fabric. I shape the decorated slab over it, and then lower the original hollow mould over them both before flipping them right way up. The slab can now be gently pressed more thoroughly into the hollow mould, and the edges trimmed with a needle tool. It can be turned out when firm for final trimming.

Other ways of decorating.
You do not have to trail. Small quantities of pouring slip can be dripped in blobs round the edge of a freshly-slipped dish, and allowed to slither down, when they can be coaxed and tilted to create marbled patterns.Fresh slip can be wiped with fingertips in striped or wavy patterns, and different effects can be achieved with wiping or scratching it away at different stages of hardness.

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