Glazes are applied by a variety of means, each of which requires a different formulation. This article covers brushing and dipping. Additives which give the required chazracteristics include gums, flocculants and deflocculants.

Parameters relating to glaze thickness and solid loading are described in my article on quantifying glaze parameters.

Dipping can be one of the fastest methods of glazing a pot. A dipping glaze is formulated to form an even coat on the piece which dries quickly without dripping or running. This is achieved by controlling the solid loading and state of flocculation.

Good dipping performance depends on adequate suspension of the glaze materials. Kaolins and ball clays make good suspending agents but they do vary considerably in this ability. Where whiteness is important good choices are Grolleg china clay and Edgar Plastic Kaolin (EPK). Of the two EPK usually performs better but for glazes that are sensitive to titanium dioxide, Grolleg is preferred. Not all ball clays suspend well and testing is required. The amount of clay should ideally be around 20% of the dry ingredients, more can cause drying cracks. Glazes made with feldspar that are lacking in clay can often be adjusted by substituting some feldspar for a high alkali, low alumina frit such as Ferro 3110.

Good results are achieved at intermediate solid loadings, a value of F0.53 corresponding to 90g of water to 100g of dry material is a good starting point. This usually results in a glaze density in the region of 1.45 g/ml.

Mixed with this amount of water, most glazes will be very fluid – too fluid for good dipping performance. The important step is to adjust the state of flocculation with calcium chloride solution. This extremely potent flocculant needs to be added very carefully. A 10ml syringe is a good method to use, it allows careful drop by drop addition. The amount required varies from glaze to glaze, typically around 6 drops (from a 10ml syringe) to 100ml of mixed liquid glaze.

With the glaze in a container where it can be stirred, the adjustment is performed by adding the calcium chloride dropwise, gently stirring to give a swirling motion, and measuring how long it takes for the glaze to stop moving. The effect of the addition is almost instantaneous. As more is added the glaze will take on a creamy consistency and the reaction to stirring will be a few seconds of motion with an abrupt stop and slight rebound. The rebound is a sure sign that the flocculant is working. If too much flocculant is added the glaze will stop moving immediately and this does not give good dipping performance. A time of around 2 – 4 seconds seems to give the best results.

Bisc ware dipped with a glaze prepared this way will take up an even coat that dries very quickly and without dripping.

Some glaze mixtures contain their own flocculants and produce a liquid which is already flocculated. Zinc oxide that has been stored in a damp environment often does this as do many materials marketed as ‘magnesium carbonate’. This can sometimes be remedied by adding a few drops of dispex to give a very fluid slip before proceeding as above to add calcium chloride.

Glazes applied in this way form an even but fragile coat. Lacking gums to act as hardeners, the layer is easily disrupted and is not suitable for layering glazes on top. If this is required then the convenience of dipping will have to be sacrificed. CMC glaze binder solves this problem but even in very small additions, prevents a dipped glaze from applying well.

I divide brushing glazes into two categories, those with high and those with low solid loadings. The second category is most useful for many potters, this is the method used by most commerical glazes marketed for brushing. Their application characteristics are similar to wall paint: they dry  slowly and allow even coats to be applied and they also perform well with multuiple layers. High solid loading is very relevant to crystalline glazes, it is the main method that I use in my work but is probably of limited interest to most potters.

A brushing glaze needs to dry slowly enough to allow an even coat to be built up. It must not be too runny or it will be difficult to hold on the brush and will move after application. A glaze prepared without additives will generally lack both these properties. Applied to bisc ware with a brush, the glaze would typically dry almost instantly sticking the brush to the piece.

The two additives CMC and Vee gum T, used in cmbination, can achieve this. CMC mainly controls the rate of drying and also hardens the glaze layer. Vee gum T mainly acts to increase the viscosity of the glaze. They do not act entirely independently, each contributes to retarding the drying and increasing the viscosity. By selecting the correct amounts of each it is usually possible to formulate a glaze that gives very good brushing characteristics.

 I have formulated many glazes this way and most work well with 1.5% each of CMC and Vee gum T with a solid loading of F0.46. Here is how to calculate the amounts for 100g of dry glaze powder.

 First, the amount of water. The target solid loading is 0.46 and therefore the amount of water required, w, is given by:

 0.46 = 100 /(100 + w)

w = (100 – 46)/0.46 = 117g

 The CMC is added as CMCSOL40 which has a mass fraction of CMC of F0.0385, the amount needed is therefore

 1.5 / 0.0385 = 39g

CMCSOL40 has a mass fraction of water of F0.962 and 39g therefore sources

 39 x 0.962 = 37.5g water

 Similarly for the Vee gum T, using VGTSOL80 which has mass fractions of F0.0741 and F0.926 the amount needed is:

1.5/0.0741 = 20.2g

and this sources

20.2 x 0.926 = 18.7g of water.

 The amount of additional water needed to give the overall solid loading is therefore 117g – 37.5g – 18.7g = 60.8g

 All these weights can be rounded to the nearest gram when dealing with 100g or more of dry glaze powder.

Mixing in a ball jar

 I mix my glazes in a ball jar. This has the advantage of producing some grinding effect that can eliminate problems due to oversized particles. To maximise this, I mix the glaze in two stages. With the above example.

 Stage 1. Add 60.8g of water, all the 100g of glaze powder and the 20.2g of VGTSOL80 and mix in the mill for about 30minutes. The result should be a glaze that is very fluid, certainly too fluid to hold easily on a brush. If it is viscous it will be due to the presence of a component flocculating the glaze (for example zinc oxide or magnesium ‘carbonate’) and this can usually be corrected by a few drops of dispex.

 Stage 2. Add the 39g of CMCSOL40 and resume mixing for another 30 minutes. Immediately after the addition the glaze is often so viscous that it is gelled completely and there can be a temptation to add more water. However, the glaze will thin quite dramatically with continued mixing.

 Mixing in a blender

 Using a blender, all the ingredients can be added at once and blending continued until a smooth glaze is obtained. The disadvantage is that the grinding effect provided by the ball milling is lost.

 The glaze is now ready to apply by brush. If the performance is poor adjustments can be made to the next batch.

Fluidity – adjust with water content, more water, more fluid.

Drying time – adjust with CMC content, more CMC increases the drying time.

Gelling tendency – add more Vee gum T to increase gelling.

This is the method I use for all my zinc silicate crystalline glazes. It is suitable for glazes that need to be applied by brush very thickly, in the case of my crystalline glazes, typically over G1200 for a vertical surface. The high solid loading makes it easy to build up a thick coat rapidly without having to wait for intermediate coats to dry. A typical formulation is:

 100g dry glaze powder

32g water

0.5g dispex

This gives a solid loading of F0.76 and typically a density of over 2.0g/ml. These are much higher values than are typically encountered in studio pottery and are only possible because of the deflocculating effect of the dispex. Without it, the glaze would not even mix into a fluid, it would form a clay-like mass.

An example is my mid blue/purple glaze. This I mix for around 30minutes in a ball jar giving a very fluid glaze, density 2.04 g/ml and which drains a Ford #4 cup in 22s (the same cup drains water in 10s). It can be applied directly to bisc ware without cracking but for improved brushing, it is better to add CMCSOL40 to harden and retard drying. A typical addition is 10g of CMCSOL40 to 100g of mixed glaze. This gives an overall solid loading of F0.69 and CMC content equivalent to 0.5% of the weight of dry glaze.