Making glazes Linda Bloomfield begins a new series on the science behind glazes lazes can be daunting, but making glazes is actually quite similar to cooking. G Potters usually follow glaze recipes they find in books or online. The ingredients come as white powders: ground flint, whiting, feldspar and clay. The powdered materials are weighed out carefully and added to a container half full of water, then left to slake, until the water has completely wetted the powder. The glaze can then be sieved through an 80 mesh potters’ sieve (meaning it has 80 holes per linear inch). Once the water has been adjusted until the glaze consistency is somewhere between milk and single cream, either by adding water or leaving overnight to settle and removing water, then it is ready to use. Like cooking, it is possible to make a reasonable glaze without much knowledge. It is important to measure the glaze ingredients very carefully with accurate scales and to make a note of the recipe and then number the This is pure silicon dioxide (also known the melt. Clay contains aluminium corresponding test tile. However, to as silica) and is a glass former. The oxide (alumina) and silica. China clay, really gain an understanding of the melting point of silica is too high to ball clay or bentonite can be used as science behind glazes means you can melt in a kiln, so fluxes are added to glaze materials. Clay also helps to have much more control over what reduce the melting temperature. The suspend the heavier ingredients in comes out of the kiln. main flux in stoneware glazes is water in the glaze bucket, preventing feldspar, which contains sodium them from settling. Glaze materials and potassium oxides. These The main material used in are alkali metal oxides, Material Property glazes is silica, in the form which react with the of ground flint or quartz. acidic silica and break Silica (flint, quartz) Glass former down the structure, Flux (feldspar, Melter helping it to melt. whiting) However, a simple Alumina (clay) Stiffener mixture of sodium and silica – sodium silicate – would make a glaze that was soluble in water and likely to wash off, so a second flux is added to make the fired glaze insoluble. This Chalk from secondary flux is whiting – calcium south-east carbonate – found in chalk and England limestone. It helps to strengthen the and Quartz from rock vein. Circled: limestone (Cornishstone) Granite from Cornwall, glaze and makes it more stable. The resulting glaze would be very runny, so from the containing potash feldspar (pink- Alps. white crystals). clay is added to make it more viscous in . Issue 4 ClayCraft 37 GLAZE SCIENCE GLAZE RECIPES Stoneware glaze recipe, transparent glossy (1260°C-1280°C) ● Potash feldspar 27 ● Whiting 21 ● Quartz 32 ● China clay 20 Earthenware glaze recipe, transparent glossy (1060°C-1100°C) ● Calcium borate frit 39 ● Soda feldspar 27 ● Whiting 5 ● Quartz 2 Clay and powdered bentonite. ● China clay 6 forming crystals in the glaze bucket that will not dissolve back into the In the earthenware glaze recipe, some glaze. A frit combines soluble materials frit has been added. This is a kind of with silica, which prevents them from man-made feldspar, containing fluxes dissolving in water. The more frit that such as calcium and boron oxides, as is added to a glaze, the lower the firing well as silica. The reason for making temperature will be. Mid-range glazes frits is that some glaze materials, such (between stoneware and earthenware as borax, are water soluble, eventually firing temperatures) can contain between 10 and 30% frit, while earthenware and raku glazes can contain up to 90% frit. Glaze properties To make a shiny glaze, the glaze materials are combined together in a eutectic mixture. This is the lowest melting combination of materials. For silica and alumina, the eutectic ratio is 9 parts silica to 1 part alumina; this mixture melts at a lower temperature Crazed glaze on stoneware dish. than either of the pure materials. Borax frit (containing sodium, Alumina is found in clay, together with alumina to two silica and two water calcium borate and silica). silica (a clay molecule is made of one molecules). Alumina is also present in feldspar (one alumina to six silica and one soda molecule). Glazes with a ratio MATHS MADE EASY of 1:9 alumina to silica (plus enough ● The weights for each ingredient add up to 100 parts in total; ie, they are percentages. fluxes to melt them) will be shiny and ● One kilo of dry glaze powder plus 750ml water will make roughly 1.25 transparent at stoneware temperatures. litres of wet glaze, depending on how thick you want it. ● Simply scale up the weight proportions to give the final liquid volume that Altering the glaze you want. To make the glaze more matt, more For example, if you assume the weights below are in grams, the recipes clay can be added until the ratio of will make 125ml of glaze. Perfect for testing, but you may want more to silica to alumina is 1:5. actually apply it to a pot. ● For 1kg of powder, multiply the weights by 10, giving 270g of potash Another way to make a matt glaze is feldspar, 210g whiting, 320g quartz, and 200g China clay, which will make to add more calcium or magnesium in approx 1.25 litres. the form of dolomite. However, these ● To make 5 litres, multiply the dry weights below by 40 (to give 4kg) and matt glazes will have subdued colour. add to 3 litres of water. To make brightly coloured matt glazes, add barium or strontium 38 ClayCraft Issue 4 GLAZE SCIENCE TrOUBLESHOOTING SUmmARY ● To correct crazing, add 5% silica, borate frit or talc. ● To correct shivering, add 5% feldspar, or reduce silica by 5%. ● To prevent crawling, apply glaze more thinly, reduce clay content, or sponge biscuit ware. ● To smooth pinholes, soak for 15-30 minutes (hold peak temperature) at the end of firing. ● To avoid blisters, fire to a lower temperature, or apply glaze more thinly. Crawled glaze on red earthenware dish. carbonate. These will give bright will appear in the glaze when the pot is in the glaze will also help. turquoise with copper oxide, or lime removed from the kiln. This is known Crawling is a problem that can occur green with chromium oxide. as crazing. It is not desirable on when the glaze is applied too thickly To make the glaze more shiny, more functional ware, but some potters use it and cracks on drying. It can be feldspar or frit can be added. as a decorative effect. To correct corrected by applying the glaze more To make the glaze melt at a lower crazing, a low expansion material can thinly, or reducing materials with high temperature, the clay in the recipe can be added to the glaze. Boron oxide has drying shrinkage such as clay and zinc be reduced. a very low expansion and can be added oxide. Materials with high surface in the form of borax frit or calcium tension when melted can cause the borate frit. Another glaze material with glaze to form beads with bare patches Satin matt glaze (1240°C-1260°C) low expansion is talc (magnesium in between. These materials include ● Soda feldspar 44 ● Dolomite 23 silicate). zirconium and tin oxide, which are used ● Whiting 3 The opposite of crazing is called to opacify the glaze, making it white. ● Quartz 23 shivering. The glaze contracts less than Crawling is sometimes caused by ● China clay 6 the clay body during cooling and starts greasy or dusty biscuit ware. If you to flake off rims and edges of handles, think this is the case, the ware should or can even crack the pot in two. This be sponged and left to dry before Looking at the glaze recipes given is a serious fault, which can be glazing. above, the differences between corrected by adding materials with Other problems are caused by stoneware and earthenware glaze high expansion, containing sodium and underfiring or overfiring the glaze. recipes can clearly be seen. The potassium, including feldspar and Underfiring can cause pinholes, while earthenware glaze has less clay, quartz high-alkaline frit. Reducing the silica overfiring can cause blisters in the and whiting. Potash feldspar has been glaze. Soaking or holding the peak replaced by soda feldspar, and borate temperature at the end of firing can frit has been added to decrease the heal pinholes. Blisters can be ground firing temperature. Soda feldspar gives down and re-fired. a more fluid melt than potash feldspar, but the glaze will be slightly softer and Linda Bloomfield is a scientist will scratch more easily. In general, the turned potter. More glaze recipes higher the firing temperature, the can be found in her books, Colour harder and more scratch-resistant the in Glazes, (A&C Black, 2012) and glaze will be, and the harder and The Handbook of Glaze Recipes stronger the fired clay body becomes. (Bloomsbury, 2014). Her next book is Science for Potters (The American Ceramic Society, 2017), GLAZE FAULTS out later this year. If the glaze contracts more than the lindabloomfield.co.uk clay body during cooling, then cracks Moon jar with satin matt glaze on porcelain, by Linda Bloomfield. Issue 4 ClayCraft 39.