Quite rightly, concerns exist around the safety of black clays in the studio and on functional ware.

It is true that historically, some black bodies have been coloured with Manganese Oxide which is a toxin.

NIGRA 2002 sold under the Primo-Pro label is manufactured by a reputable European supplier and widely sold throughout Europe. We have our supplier’s absolute assurance that this body (pigmented with manufactured stains) is safe to use.

The glaze composition is largely determined by the effects that characterise that glaze. Consequently, some glazes suspend easily in the bucket and others need help. The first rule is to avoid over-watering the glazes as there is no way that a very thin glaze will suspend well. Some glazes with very low clay content can be treated with bentonite and a flocculant to aid suspension. Our preferred flocculant is plaster of paris – it is cheap, most of us have some in the studio and the effect is long-lasting. Alternatives are Epsom salts and calcium chloride. Pre-disperse the flocculant in water and the resulting slurry gradually to the mixing glaze. As a rule of thumb, you might use one or two teaspoons of plaster per bucket of glaze. Adding bentonite(in similar quantities) can also contribute to improved suspension and better-bound glaze surfaces. Add the dry bentonite to water, blitz with a milkshake maker (or mix by hand and stand overnight) and also add gradually to the glaze until the right smooth creamy texture is observed. This sounds complicated but it becomes second nature after a while.

If your glaze has settled into a hard layer with clear water on top, first scoop off (and reserve) most of the clear water. The glaze can then be (manually) redispersed in the remaining water to the desired consistency. Add back as much of the reserved water as needed. Glazes that have settled like this will almost certainly need the bentonite/ plaster treatment to keep them suspended. They should have the consistency of yogurt when they have been allowed to stand for a few days but should be easy to redisperse with hand mixing.

It is important to remember that crazing (and its opposite fault, shivering/chittering) reflects mismatched clay and glaze combinations. So – the first solution may be to look at your material selection and the reasons for that selection.

There are possibly thousands of different glazes in use in Australia, and scores of different clay bodies. Each has a useful firing range within the material may give a satisfactory result but where the result will likely vary with changing temperature. Each material has a unique COE (coefficient of expansion), that is to say that each fired glaze or body expands and contracts at a rate that reflects its composition and the firing process.

Glaze fit is not an issue at high temperatures where the body may be softened, and the glaze is a fluid. As the piece is cooling in the kiln it will reach a temperature (different for all glazes) where it freezes or solidifies. At this point both glaze and body are solid and shrinking as they cool further in proportion to the COE. If the COE of the glaze is higher than the fired body the glaze will probably craze as it cools due to the stress between the two solid materials. A small COE differential may result in slight crazing (indicated by few, widely spaced crazing lines) and possibly only long after the kiln has cooled. Consider all the antique pottery with heavy crazing which has developed over years or decades. A large COE differential may result in a fine crackle, usually visible when the kiln is opened or soon after. Crackle glazes are examples of deliberately causing crazing with a high COE glaze. Even in this case the crazing effect will be different across a range of bodies.

Crazing is not the only fault that may be triggered by glaze fit issues. If the COE of the glaze is much lower than that of the clay body, then it is the body that will be under tensile stress on cooling as it shrinks more than the glaze. It is beneficial for the body to be under modest tensile stress as this keeps the glaze in slight compression, enhances the strength of the piece and resists crazing even when subject to rapid temperature changes. In severe cases where the body is under tension and the glaze under compression, glaze may chitter or shiver off the pot – usually on the edges, spouts, and handles. Less severe cases may be prone to dunting (cracking when exposed to thermal shock). Mugs prone to dunting may crack when filled with boiling water.

Whilst it is logical that these faults can occur, they can be managed. Your options are to review your material selection or to modify the materials. Unless you are using casting slip, there is no easy way for you to adjust the body, but you can adjust the glaze. Some visual glaze characteristics may change as the result of such adjustments.

Adjusting a glaze to reduce crazing:

If your glaze is crazing and if it is bright and glossy it will likely dissolve more low COE materials in the melt.

Silica: Increasing silica IN A GLAZE reduces the COE and crazing risk as long as it dissolves.

Kaolin: Increasing kaolin in a glaze will also reduce the COE. It may have the secondary benefit of improving suspension in the bucket.

Talc: Adding Talc to a glaze will usually lower the COE. In excess it may have a matting effect (at which stage it is developing crystals rather than dissolving) and the magnesium in talc can affect some colours.

Boron: Adding a high boron frit, usually as a minor component with the other materials mentioned can improve the solubility of those materials in the glaze. By themselves, most frits will increase the COE.

These three materials can, of course be added in combination with one another. Potters wishing to make these adjustments should do so incrementally and keep a careful record of both adjustments and outcomes.

Adjusting glazes to reduce/eliminate dunting:

Here we are looking for high COE materials that can be added to increase the overall COE of the glaze. Again, there may be impacts on the visual result. A benefit of this is that an adjusted glaze is uniquely yours. Again, the opportunity to make effective adjustments will depend on the ability of the glaze to dissolve or incorporate the modifiers.

Our best high-COE options (which may be used in combination) are:

Potash Feldspar

Nepheline Syenite

High alkaline frit – eg Frit 4110

Calcium Carbonate/whiting.

If you are experiencing crazing with your favourite glaze and clay body then it may be effective to perform the adjustments suggested, or even increasing the firing temp somewhat. If the glaze continued to craze – ask yourself if it really matters to you.

Safe handling of potters' materials is critical to minimizing health risks in both hobbyist and professional pottery studios.
While clay and glazes are central to the craft, many of the materials used can pose health hazards if not handled properly.

Dust Control: The dust from clay, glaze materials, and other pottery ingredients often contains silica, which can cause lung disease, including silicosis, after prolonged exposure. Potters should ensure their workspace is well-ventilated, use wet cleaning methods to minimise dust, and wear appropriate respiratory protection (e.g., N95 masks) when mixing or handling dry materials.

Chemical Safety: Some glazes and colouring agents contain heavy metals, such as lead, cadmium, and manganese. Exposure to these can lead to serious health issues, including nervous system damage. Gloves should be worn when handling these materials, and thorough hand washing after use is essential.

Proper Storage: Materials should be stored in clearly labelled, sealed containers to prevent accidental exposure and reduce the risk of inhalation. Keeping hazardous materials away from food preparation areas is also crucial to prevent contamination.

Personal Protective Equipment (PPE): In addition to respiratory protection, potters should use gloves, aprons, and eye protection when handling chemicals or working with dry materials.

By following these safety practices, potters can enjoy their craft while minimising health risks.