Linda Bloomfi eld

GLAZE BASICS: ALTERING YOUR GLAZE

Last month, I covered how to make and fire glaze tests. This month I will explain how to analyse your glaze test results and adjust the glaze or correct any defects

Linda Bloomfield is a scientist turned potter. More glaze recipes can be found in her books, Advanced Pottery (2011), Colour in Glazes (A&C Black 2012), The Handbook of Glaze Recipes (Bloomsbury 2014) and Science for Potters (The American Ceramic Society 2017). Her new book Special E ect Glazes, and the second edition of Colour in Glazes are both out now. Linda has recently launched her online glazing course, How Glazes Work, on her website:

lindabloomfield.co.uk

Above: Glaze defects: Crazing, crawling, blisters and pinholes. Above: Hollis Engley’s crawled shino bowls. The white shino glaze has crawled, showing the stoneware clay beneath. Shino glaze is high in clay, which can cause crawling.

Once you have fired your glaze test and unloaded it from the kiln, you can begin to use your new glaze, make up a bigger batch and glaze some pots with it. Never throw a glaze test away; you may want to come back to it later. After your initial excitement or disappointment, it’s time to take a closer look and analyse your results. Think about the following qualities.

Is the glaze glossy or matt? Is the glaze runny? You will only have an idea of how much it runs if the test tile was fired vertically. Is it the colour you expected? Are there any visible glaze defects: cracks, pinholes or blisters?

Glaze defects

There are several glaze defects that can occur, although one potter’s defect might be another potter’s special effect! One such defect is known as crazing, when a network of fine cracks develops in your glaze, accompanied by a series of pinging sounds. Crazing occurs when the expansion of your glaze is too high compared to the expansion of the clay body. As the glaze cools after firing, it contracts more than the clay body, and cracks develop to release the stress. If this effect is desired, it is called crackle, although it greatly weakens the pot, which will break more easily than a pot without cracks in the glaze. If you don’t want crackle, you can adjust your glaze by adding quartz (silica) or talc (magnesium silicate) to the glaze in 2% increments, then sieving, dipping a test tile and firing to see if the crazing is reduced or eliminated. You might also need to try out the glaze on a larger test tile or pot to be sure there is no crazing. Thinning the glaze with water will also help correct crazing.

The opposite of crazing is known as shivering, where the glaze has a lower expansion than the clay body. This can cause the glaze to flake off in sharp flakes at the base, rim or edges of handles. It can cause porcelain or stoneware pots to crack in two, known as dunting. To correct shivering or dunting, you need to add a high expansion material to the glaze. Sodium oxide has the highest expansion of all glaze materials, so you can add this in the form of soda feldspar or nepheline syenite, around 2%. As well as correcting shivering, it might also cause the glaze to be slightly more runny.

Crawling occurs when the glaze forms into beads with bare patches in between. It can often be caused by applying the glaze too thickly. You can add water to the glaze to get a thinner application. Some glaze materials can cause crawling, including ball clay and zinc oxide. These materials can be calcined (heated to biscuit temperature) to reduce their shrinkage during drying.

 Dunting. These underfired plates have cracked owing to a mismatch in the expansion of the clay and the glaze.

Other glaze defects include pinholes and blisters. Pinholes can sometimes be eliminated by firing slightly higher, by soaking for 15-30 minutes at top temperature, or by firing the biscuit ware to a slightly lower temperature. For the biscuit firing, I usually fire to 990°C (cone 06). The lower the biscuit firing temperature, the more absorbent the biscuit ware will be during the glaze application, and the fewer pinholes there will be.

Blisters can be the result of overfiring or the glaze containing a problem material such as bone ash or silicon carbide, both of which give off gases during firing. If your glaze is too runny, it may form a pool at the bottom of the pot, which might also cause blisters. If this is the case, you can grind down the blisters and refire.

Adjusting the glaze

Is the glaze dry and rough? It might be underfired. If you would like the glaze to be more glossy and runny, you can add a small amount, around 5%, of a frit, such as borax frit (or Ferro frit 3134). This will help your glaze to melt better and is a good way to convert a high temperature (cone 10) stoneware glaze to a mid-temperature (cone 6) glaze.

Is the glaze too runny? You can add more china clay to stiffen it. Try increments of 2%. Adding china clay might also make the glaze slightly less glossy. Conversely, if you want your glaze to be more runny, you can reduce the amount of clay in the recipe or add a small amount of borax frit.

To convert a matt glaze to a glossy glaze, you can add more quartz (silica). Try adding 5, 10, 15% quartz, sieving and dipping a tile each time so that you get a series of glazes with increasing gloss. Eventually, if you add too much silica, the glaze will become sugary and underfired, as silica has a high melting point, and beyond a certain amount, it will need more fluxes to melt it. To make a glossy glaze more matt, you can either reduce the silica or add more china clay or dolomite in 5% increments.

Above: Crazing/crackle in an ash glaze.  Glaze tests made by Cornish potter Julia Crimmen using chromium, tin and copper oxides.

Colour

Colouring oxides added to the glaze will be affected by the fluxes in the glaze. In general, magnesium (talc, dolomite) and zinc oxide will mute some colours and destroy chrome-tin pinks. A lack of calcium (whiting) in the glaze will also prevent the chrome-tin pink colour from developing. Barium, strontium and lithium carbonate will brighten colours, particularly turquoise from copper and chartreuse green from chromium oxide. You need to choose the right glaze base for the colour you want to develop.

If you would like your glaze to be more interesting, try adding 5% rutile, which will give streaks and mottling. In matt glazes, a small amount of zinc oxide will help to grow tiny crystals in the glaze. 

 Set of glaze tests containing 5% rutile and various colouring oxides: copper, chromium, iron, manganese and tin oxide.

For glaze recipes, try books such as Colour in Glazes, The Handbook of Glaze Recipes or search online on glazy.org