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Capturing an Image

Technical | Stoneware Glazes

Greg Daly develops a stoneware palette

t can be quite easy. Taking a stoneware base recipe, in this case Bernard Leach’s Cone 8 recipe, I will make alterations to develop two distinct glaze palettes. The seascape image (see below) will form the basis for both surfaces and colours.

How do you go about creating your own surface images with glaze from a passing image you have seen or something you’ve collected? The inspiration may be very simple: a black stone, partly wet; the colour of a leaf – pinks, greens, browns and greys; the pastel colours of a fabric you are wearing.

My starting point is to carefully observe what I see in the colour and texture. Is the surface glossy, satin or matte? How many tones of a colour are there? Is the colour even, or does it change with patterns or movement?

The image selected will be the initial basis for the development of glazes. For this example I have chosen an image of a rock pool where there are two distinct colours and surfaces. Firstly, there is the water with its colours, blue and turquoise and the movement of the water’s surface. A high alkaline copper glaze will be needed for the turquoise, along with a cobalt blue glaze. Applying these over each other should give the desired effect. Secondly, there is sandstone, covered with a seaweed growth, with its matte surface of sandy brown moving through to the mustard green of the seaweed; two quite different surfaces and colours. Each could be developed individually, but I am drawn to the contrast of these two opposites.

Above:The initial image for the development of Greg’s glazes

There are a large number of base glazes one can select from and develop. I have chosen Leach’s Cone 8 (1260ºC) glaze as a base. This glaze can happily fire up to cone 11 (1315ºC) for a glossy glaze, or fire lower to cone 6 (1222ºC) for a satin glaze.

Leach’s Cone 8 (1260ºC)
Potash Feldspar 40
Silica 30
Whiting 20
Kaolin 10

This recipe will be used as a framework for my two different base glazes. Leach’s glaze with 3% cobalt carbonate will give a good dense blue; while adding 3% copper carbonate will produce a green (see Figure 2).

By applying the fluid turquoise glaze over the Leach Blue and Green glazes we achieve the desired, watery surface. This effect could not be attained if the glazes were not combined. To make the Turquoise Glaze I will substitute barium carbonate (an alkaline flux) for the potash feldspar. I will also use soda bicarbonate (a high alkaline, low temperature flux) or alkaline frit 4110 instead of whiting.

N.B. Barium needs to be handled safely. When mixing any glaze, use a good dust mask and, with toxic materials, use disposable latex gloves.

With 2, 4, 8 % of copper additions we have a palette of greens (Figure 2). If the glaze is too fluid, try Glaze 1a. The higher percentage of silica will not change the colour, but will increase the melting temperature.

For the Matte Sandstone Glaze I will also use Leach’s glaze as the base, but will add equal parts of Alumina Hydrate (I have used 80 mesh. Other forms of alumina are suitable, including calcined alumina). This will give a matte, but silky, touch to the glaze.

With addition of iron oxide 2, 4, 8, and 15 % to this base, a wonderful colour response in the high alumina glaze is achieved. With vanadium pentoxide 10% added, we have the seaweed green. (Figure 2) All the glazes were fired to cone 9, oxidised.

For interest, you can line blend alumina into the Leach base glaze (or any other base glaze), using 10, 20, 30, 40, 50, 60, 80% of alumina. You will find an interesting progression of both surface and colour response. Keep the oxide constant, e.g. iron 10, or cobalt 3. The resulting blue change with cobalt is very interesting, from dark blue to light sky blue.
N.B. Light blue commercial stains are cobalt/alumina-based.

Glaze 1 (Turquoise) Barium Carbonate 40
Silica 30
Soda Bicarbonate 20
Kaolin 10

Glaze 1a (variation)
Silica 40
Barium Carbonate 30
Soda Bicarbonate 20
Kaolin 10

Glaze 2 (Sandstone)
Potash Feldspar 40 20
Silica 30 15
Whiting 20 10
Kaolin 10 5
Alumina Hydrate 100 50

Glaze 1 +2	+4	+8 copper Leach	+3 copper	+3 cobalt

Glaze 2 +2	+4	+8	+15 Iron	+10 Vanadium

I now have a selection of stoneware glazes with which to work. By testing them under and over each other, I can gain the information needed to achieve the tones, textures and colours seen in the source image.

1. Take a tile (not too small or the results will not be clearly seen).
2. Arrange, in a line, the containers of the glazes to test.
3. Mark the test numbers in the same order down the back of the tile.
4. Paint each glaze in a line across the tile in the same order.
5. Turn the tile 90º and repeat the order across the tile. This way it is possible to see what each glaze is like under and over each other. (N.B. Put a labelled arrow, using ‘U’ for under and ‘O’ for over, on the back of the tile to indicate the layering order) Refer to Figure 3.

I now have a pallette of coloured glazes and surfaces with which to start working. I need to consider the application of these glazes to give the impression of the water and rocks. Looking at the image, it is noticeable that there is no flat colour anywhere; it is made up of multiple colours and movement. The blue and turquoise water has movement on its surface. To capture this, use a brush to dab and wash the cobalt and copper Leach-based glazes in areas. This allows the underlying colours to come through and affect the turquoise glaze on top. Don’t be too careful when applying the glaze; variation of thickness is desirable. Next apply Glaze 1 Turquoise (with 2% copper) all over the water area (allow some of the blue and green glaze to show through). I chose also to use Glaze 1 with 8% copper as an extra highlight. Again, try using an uneven painted application to create the water effect.

The dry sandstone area is also applied this way using Glaze 2 with 1, 8, and 15 % iron. Applying the dark iron first (i.e. underneath) allows the darker colour to come through the lighter tan glaze (see grid test Figure 3). If the dark was applied on top it would take over and be too raw a colour. Finally, apply the green moss vanadium glaze as a highlighter. I used 1.5 to 7.5 cm paint brushes.


Left: Application tile Right: Finished platter (detail)

Don’t launch straight into glazing pots, as practice is needed to develop a new idea. Use tiles to practise applying the glazes (cut straight from a packet of clay, then bisqued ready for use). This way you will get valuable feedback, allowing more confidence and freedom when glazing the final piece.

To develop an idea from the first image to a final piece, it is essential to go through the testing of base glazes and colour – grid tests first, then use a play tile for working out application and composition, before moving on to the final piece. Each step will give important feedback for the next stage.
It is important to ‘play’ along the way
Remember, the final work may have no relationship to the original starting point. Glaze recipes are not set; they can be changed and altered. Try taking your favourite old stoneware glaze and adding alumina to it, or take out one of the ingredients and substitute another flux.

As you can see from this example, an all time standard glaze base can become two very different glazes.

# Note: Cone temperatures quoted use a rise of 150ºC per hour. As cones measure the work done by the heat in the kiln, the cones will bend over at different temperatures. For example, Cone 9 fired at 60ºC per hour, will melt over at 1260ºC; Cone 9 fired at 150ºC per hour will melt at 1280ºC; i.e. the temperature will be higher for faster heating rates and lower for slower heating rates. When firing, especially stoneware, always use cones as your final guide. Pyrometers measure the air temperature in a kiln.

Cone chart: www.ortonceramic.com/resources/pdf/wall_chart_horiz.pdf