The effects of washing ash

Ash contains soluble alkalis, which, when mixed with water, make the glaze suspension very alkaline and therefore caustic with obvious dangers when handling. The soluble alkalis are however strong fluxes and some of the useful components of the ash are therefore lost through washing.

As mentioned in Chapter 2 (p.34), opinions are polarised amongst practitioners as to whether or not ash needs to be washed before being used as a glaze ingredient. Opposing opinions are demonstrated within the case studies carried out for this research. Dodd is convinced that it is essential to wash ash

otherwise glaze faults will occur, due to soluble alkalis penetrating the clay body as it absorbs the water of the glaze mix. Conversely, Mellon never washes ash and has never encountered any problems. In fact he is keen to retain all the soluble fluxes in order to keep the level of calcium as low as possible in glazes to be used over his oxide drawings, having found that high calcium levels cause the drawings to run (Mellon, 1976, 1977, 1980, 1998 & 2000a). Potassium is therefore a very valuable flux for producing a stable glaze.

Using figures from the EDX analysis, the effect of washing on the composition of ash can be evaluated. Three types of ash, bean, pea and linseed, have been tested both washed and unwashed. The chart in Figure 24 (p.47) shows the percentage concentrations of the main fluxes detected in the washed and unwashed ash samples and demonstrates just how dramatically washing alters the balance of the fluxes within the ash. The washing process hugely reduces the concentrations of potassium and consequently the percentages of calcium have risen. Potassium oxide is largely soluble in water and is therefore lost when the water is poured off the ashes during washing. This change in the balance of the fluxes present will significantly change the characteristics of the glazes produced. Before washing, the flux concentrations in the three ashes

tested are very different from each other. After washing much of this variety is lost and all three are quite similar.

Figure 24:

Chart showing the effect of washing on the flux content of ash

Table 5 overleaf summarises all the arguments for and against washing ash;

those gleaned from the contextual study, those from direct contact with the ceramicists Dodd and Mellon through the case studies, as well as findings from my own practice.

COMPARISON OF WASHED AND UNWASHED ASHES

0 10 20 30 40 50 60 70 80 90

Unwashed Pea

Washed Pea

Unwashed Bean

Washed Bean

Unwashed Linseed

Washed Linseed ASHES

PERCENTAGE CONCENTRATIONS OF FLUXES

Potassium Calcium

Table 5:

Summary of the arguments for and against washing ash

REASONS TO WASH ASH REASONS NOT TO WASH ASH

 Unwashed ash contains soluble alkalis, which make the water in the glaze alkaline and therefore caustic to handle. Tests carried out in this research showed it to be pH 11 or more. Gloves should be worn.

 After 3 or 4 washings the ash was still very alkaline, testing at pH 10.5 – 11, so a great many washings would be needed to remove all the soluble alkalis.

 Repeated washing is very time consuming.

 Soluble alkalis penetrating the clay body in the water from the glaze could cause glaze faults when fired. Dodd has occasionally had glaze faults which he has attributed to insufficiently washed ash

 Mellon, who has worked with ash for 40 years, never washes ash and has encountered no

subsequent glaze faults.

 The soluble alkalis removed by washing are mainly potassium and sodium, which are strong fluxes, useful in glazes. Some of the fluxing power of the ash is therefore lost through washing.

According to Rogers (1991, p.36) it can be as much as 25%

Continued overleaf

Table 5 contd.

Summary of the arguments for and against washing ash.

REASONS TO WASH ASH REASONS NOT TO WASH ASH

 According to Rogers (1991, p.36) soluble alkalis in the glaze slop can cause problems if the glaze is to be kept for lengthy periods

 Mellon regularly keeps glazes containing unwashed ash in sealed containers for around 5 years, sometimes longer, with no problems

 In this experimental work, glazes have been kept for several months without difficulty

 As shown in the chart in Figure 24, after washing, the

compositions of the ashes are all quite similar. Much of the variety between them is lost when the soluble fluxes are washed out

 Sieving the ash mixed with water is safer as there is no dust.

 Sieving the dry ash proved easier than expected. When ash is sieved within a plastic box with the lid over as far as possible, very little dust was created. A mask was of course worn.

On consideration of the data provided by the EDX analysis and the arguments above, in particular the time taken to wash the ash, the loss of variety and the evidence of significant loss of fluxing power, the case for not washing the ash appears to be the stronger. I made the decision therefore that so long as no major problems subsequently developed, this research would concentrate on the unwashed ashes, supported by selective sampling only of washed ash.