Colouring oxides are added to glazes to produce colour. They dissolve in the glaze to produce transparent glazes with more depth than those made using commercial stains. I prefer to add only small amounts of colouring oxides to give pale, watery transparent glazes.
The glaze recipes are in my book The Handbook of Glaze Recipes. The book shows how to make a wide range of subtly coloured glazes predominantly using the colouring oxide rather than commercial stains.
My next book Science for Potters is coming out later this year.
Earthenware glazes are made mostly of frit, which is a kind of man-made feldspar. Borax, soda and other water soluble materials can be made insoluble by heating together with silica. The resulting glass is ground up and used together with clay and silica to make low temperature earthenware glazes. Frits are also used to lower the melting temperature of mid-range glazes. The raw mineral borates are found in dried up lake basins in Turkey and California.
Opacifiers can be added to make the glaze opaque. Tin oxide is the most effective opacifier but zirconium silicate is less expensive.
A respirator mask should be worn when weighing dry materials. The dry powders are added to water and left to slake before sieving.
Stoneware and porcelain glazes are made up of at least four ingredients: silica, feldspar, whiting and clay. This is what they look like in their raw mineral forms. Silica is ground flint or quartz, usually from sand or sandstone. These are large pieces of milky quartz from a vein in igneous rock.
Silica has a high melting temperature. In order to melt in a kiln, it needs a flux. The main flux in stoneware and porcelain glazes is feldspar, found in granite, an igneous rock composed of the minerals feldspar, quartz and mica. This piece of granite from Devon has both muscovite (silver, sparkly) and biotite (black, crystalline) mica. The dark vein is mafic rock, high iron and magnesium. Granite forms when molten magma cools and solidifies. The darker minerals solidify first, then the feldspar and finally the quartz, often in large veins running through the rock. Slow cooling, deep in the earth’s crust results in large crystal size.
Clay contains both alumina and silica and is added to increase viscosity to prevent the glaze from running off the pot when molten in the kiln. Clay also helps to keep the heavier quartz and feldspar suspended in water in the glaze bucket.
Calcium carbonate added in the form of whiting is an extra flux which helps to stabilise the glaze and can also produce a matt glaze surface. Whiting is ground up chalk or limestone.
Earlier this year the talented R&A Collaborations came to see me in London to make a short film about me and my work. This is the latest in their series of films about makers and their work and I’m really pleased to now have this film about me.
Over the last few months I have been solidly making tableware for restaurants, first 160 matt white plates for Luca in Clerkenwell, then 300 pieces including cake stands, plates and shallow bowls for Jamie Oliver’s Barbecoa Piccadilly. I have developed a routine. I make forty plates every Thursday, turn them on Friday, fire them over the weekend and glaze them on Monday and Tuesday. I have been doing this for about three months, sometimes making two batches in a row so that the heat from the kiln firing of the first batch dries out the second batch in the warm studio. I delivered the plates to Luca in November and to Barbecoa in January. I am really looking forward to going to the opening of Barbecoa in February and sampling the afternoon tea.
My latest article, in Ceramics Monthly September 2016, explains the science behind colour.
I am really interested in why things are coloured. When white light falls on certain objects, they absorb some colours of light and what we see are the remaining colours of light reflected from the object. Some objects, like trees and grass, use light as energy in photosynthesis and absorb red and blue light, causing them to appear green (the three primary colours which make up white light are red, blue and green). Other objects such as coloured gemstones and glazes have transition metal atoms in them, either as impurities or added intentionally. These absorb some colours of light by promoting some of their electrons to higher-energy orbitals. For example, cobalt silicate in glazes absorbs yellow light. The colour we see is the complementary colour, blue. The colour depends on the type of transition metal and the shape of the electron cloud around it, which can be different in a gemstone from in a glaze, for example, chromium impurities give a red colour in rubies but usually give a green colour in glazes. This is because the surrounding atoms in the ruby crystal are forced closer to the chromium atom than in a glaze.
Recently I have noticed a beautiful pink-grey-mustard colour combination in art, prints, textiles and interior design.
I have made a collection of porcelain bottles with the same colour combination. The bottle shapes are inspired by the still life paintings of Giorgio Morandi. The glazes are all made from raw materials in my studio. The mustard yellow comes from nickel and titanium, while the pink is made from rutile and tin oxide. These are chalky matt glazes with microscopic crystals covering the entire surface of the glaze. The pale grey is made from a combination of cobalt and nickel oxides in a dolomite glaze. Where the glazes overlap, there are interesting reactions. The dark grey box frame was made by Henry Bloomfield.
I will be showing the new bottles and vases in the British Craft Pavilion at the London Design Fair, the new name for Tent London at the Old Truman Brewery on September 22-25. Also during the London Design Festival, on 20 September I will be demonstrating throwing on the wheel at the Canvas Home showroom.
Clay is an amazing material. You can shape it and it holds its form, then you can fire it and it turns into stone. Under the microscope, clay is made up of tiny stacked hexagonal crystals, which are able to slide over each other when lubricated by water. The kaolinite image below is reproduced from the Images of clay archive of the Mineralogical Society of Great Britain & Ireland and The Clay Minerals Society.
Each clay crystal is made up of thousands of layers of silica tetrahedra and alumina octahedra. These can build up in alternating layers as in kaolin, or in three layers; silica-alumina-silica, as in bentonite. Water can get between the crystals, which enables them to slide easily over each other, allowing the clay to be moulded into shape. Kaolin is less plastic than bentonite as it has a larger particle size. A third type of clay, illite, is derived from mica, and is a constituent of red earthenware clays.
I often receive emails asking about glaze problems. It seems that students on ceramics degree courses are not taught very much about science or understanding the materials, but concentrate more on learning about art and design ideas and philosophy.
In Chemistry at school, I loved the periodic table. It neatly groups the elements so that each column contains elements with similar properties which react chemically in the same way, with metals on the left and centre and non-metals on the far right. The periodic table here shows the elements found in clays and glazes. The blank spaces are elements of less interest to potters. You can find the complete periodic table here.
I have been writing a book on Science for Potters for the American Ceramic Society. Several chapters are being published in Ceramics Monthly. You can read the full article on Chemistry for Potters here Bloomfield_Feb16
Originally published in February 2016 issue of Ceramics Monthly, p60-64. http://www.ceramicsmonthly.org . Copyright, The American Ceramic Society. Reprinted with permission.
Last week artist Sophie Glover came to my studio. She is making a large series of drawings of people working in their studios. She is really good at telling a story, showing multiple stages in the making process and picking out interesting details. I love the illustration of the wedging process and the wobbly bowl being thrown in the front centre of the drawing.
Sophie studied drawing at Falmouth University. She went to St James School in London, where her younger sister Amy was in the same class as my daughter Alice. Last year Sophie had a residency on an island near Finland, where she drew rocks and experimented with clay. She is using drawing not just as illustration, but as an art form in its own right. She is planning to make a large number of drawings of studios which you can see on her website www.sophieglover.co.uk.