Otago – Duncan Shearer

Otago Potters Club Workshop

July Online Sessions. Firing 16 September

Online Discussion of Slips and Glazes for wood firing soda glaze.

Background

The oil firing soda glazing process offers great potential to create interesting and complex surfaces. The kiln is fired to around 1300 degrees (cone 10) and the soda is introduced from about cone 7 to cone 10. It is the interaction between the oil flame produced by the firing and the addition of a powerful flux (soda ash) with the surface of the clay that results in the effects. Added to this mix is that the kiln is under reduction from about 850 degrees to the end – this alters the way different colouring oxides react. The main oxide present is iron, either in the clay, slip or glaze and can produce a wide range of colours. When we look at various clays, slips or glazes the key ingredient is silica, the amount of silica present directly translates into the amount of glass formed.

Clays

Choosing a clay to make your work from is a balancing act, on the one hand it has to behave in the making process, on the other hand it has to cope with cone 10 and provide interesting surfaces with the soda. There is no perfect clay. The easiest approach is to choose more than one clay, usually a buff stoneware and a white firing option – then use slips and glazes to modify the base clay’s response to soda. Cone 6 clays can be problematic, occasionally they melt, or dunt spectacularly after the firing. When choosing a clay I look at the Alumina:Silica ratio (the more Alumina the better) and iron content (the higher the iron level, the darker brown the clay will go).

Examples

White Stoneware clays
White Special Stoneware, Primo WM2502, Primo stoneware RW1002, Primo Pro W-White, Potclays White Special Stoneware, Potters High Fire White Stoneware, PCW, JB3, B-Mix, White St Thomas

Buff Stoneware clays
examples – Macs Buff, Primo Toast RW1802 Stoneware, Primo PRGI4002, White St Thomas, CB3, Feeney’s dark stoneware, Rods Bod

Porcelain or similar (without slip are prone to carbon trapping)
examples – Primo High Fired Porcelain, Primo PW20, Potters Clay Classic White, Hanjiki Half Porcelain, Pot White Ajoo

The documents above illustrate the value in getting an analysis of a clay if possible – allowing comparisons of Silica:Alumina ratios, iron content, flux content etc.

Slips

Slipping is one of our major methods to influence colour and surface development in Oil Firing and Soda Glazing. There are a range of approaches used depending on clay choices, decorative desires and application methods. Care is needed that the particular slip you’re using is the right density for your application and clay. Sometimes slipping is used to hide the clay body, or add decorative options, or just add more tones than what the bare clay offers.

Application methods:

Raw Slipping – applied at the hard leather-hard stage, when the clay is virtually water proof. The slips can be cream to milk in consistency (getting this right requires practice and noting down the relative density). Pouring, dipping and brushed on all work well – no spraying as there’s no absorbancy. Additional decorative options include sgraffito, finger swiping, feathering.

When slipping leather hard clay then try to have the entire piece at the same dampness.

Dry Slipping – applied at the bone dry stage. This is mostly used to create crackle slips and you definitely need to first test the thickness of the slip. Can be used to create more distinctive brushed slips. Spraying slips on is an option, care must be taken not to oversaturate and create runs.

Bisque Slipping – Spraying is your best option. Brushing and thin dips or pours are possible, test first to check for slip thickness. Tip: if you slip after liner glazing then the absorbency is reduced and it’s easier to get the thickness right.

Slip Types

Flashing Slips

Are made from clays, fluxes, silica (sometimes), opacifiers. They are best thought of as modifiers of the clay body rather than a covering that obscures the clay (Not like traditional earthenware slips), hence they are often applied as a thin coat. They can broaden the range of tones, help prevent excessive carbon trapping and offer more decorative opportunities. After bisque firing glaze can be applied over the top, usually with little change to the glaze.

Application methods on leather hard clay: Pouring, dipping, brushing.

Application methods on bone dry clay and bisque ware: Spraying.

Examples: 1:1:1, Bauer, Ohinemuri, Karangahake

Flashing Slip Recipes

Coloured Slips

Often similar ingredients as flashing slips, they also include colourants. These slips do somewhat obscure the base clay, and are modified by it. Often applied at leather hard, they can be applied after bisque (depending on the slip).

Application methods on leather hard clay: Pouring, dipping, brushing, sgraffito.

Application methods on bone dry clay and bisque ware: Spraying, some brushing

Examples: Amarillo, Jane Hamlyn’s Blue, Black Line, White Line