Last weekend I went to Burnt Earth Studio in Forest Row to teach a glaze course with potter Candice Coetser. We spent the first day mixing up a base glaze and adding colouring oxides.
Glaze course at Candice Coetser’s studio
At the end of the day we loaded the glaze tests into the kiln. Each student made 15-20 glaze tests with various combinations of five different colouring oxides.
Test tiles loaded into the kiln
We fired the kiln overnight. The next morning, I gave a talk on glaze chemistry and after a delicious lunch we managed to open the kiln at just below 200 degrees C.
Glaze tests
The glaze tests came out really well. Most students were interested in matt glazes. They tried adding cobalt, copper, titanium, nickel, manganese and tin oxides. An interesting discovery was that nickel oxide makes a light apple green in magnesium matt glazes. The favourite colour was the duck egg blue made from a combination of copper and tin oxide. There were also some interesting pinks and mauves from manganese and titanium. I will be back in Forest Row in April teaching at Forest Row School of Ceramics.
Since the end of June I have been working on a very large order for G&G Goodfellows, who are supplying a new restaurant in Woolsery, Devon.
I first met G&G Goodfellows in January 2018 when I applied to their Talent for Tableware competition at the Table show, which took place at Kensington Olympia. Table was a brand new show that was connected to trade show Top Drawer & Home, and focused solely on products that suit a restaurant environment. Since then I’ve been working with the team at G&G Goodfellows to launch a collection named ‘Simplicity’ that is exclusive to their London show room; a collection of matt white, grey and black glazed pieces, which can be mixed and matched to the client’s choice.
During the past 9 weeks, I have thrown and glazed over 300 pieces of porcelain tableware. The elements in my kilns have nearly worn out from the intense period of firing every three days; so much so, that I had to add on an additional 10 minutes soak during the glaze firing, just so the matt glaze would reach the correct temperature. Luckily, in that time, there were only three breakages; two bisque fired plates that cracked, and just one cracked glazed plate.
The whole order has been shipped off in two stages via pallet delivery. When sending a large number of boxes with fragile contents, it’s generally safer to keep all the boxes together, along with lots of bubble wrap and padding inside the boxes.
I’m looking forward to this year’s Artists at Home open studios. I will be showing new work, one-off pieces, porcelain tableware, as well as samples and seconds. The roses are in full bloom and the weather is looking good. You are welcome to visit on Friday evening 15 June 6-9pm or Saturday or Sunday 11am-6pm. 21 Flanchford Road, Stamford Brook, London W12 9ND.
This year I am sharing my studio with textile artist Ekta Kaul who makes beautiful scarves and map quilts. She will be bringing her community project Chiswick without Borders, an embroidered map of Chiswick connecting inhabitants to their roots all over the world. She will also bring her silk scarves and large map quilts.
There are several other artists showing in the area, Juliet Strong who makes jewellery across the road from me and Kate and Jonathan of Starch Green, who make prints. You can find them on the Artists at Home website or pick up a booklet from local shops and libraries in Chiswick, Hammersmith and Shepherd’s Bush.
I have been writing a regular glaze article every month for ClayCraft magazine, a new pottery magazine available from WHSmith. ClayCraft magazine includes pottery tips, projects to make by Kevin Millward and technical articles by Alan Ault of Valentine Clays, as well as a diary entry every month from Doug Fitch, who makes traditional wood-fired slipware pottery with his wife Hannah McAndrew.
In my articles so far, I have covered making a base glaze, adding colouring oxides, test tiles, applying glazes, ash glazes, special effect glazes, setting up a studio and how to correct crazing. In the next issue, my article will be on barium and strontium matt glazes with recipes for strontium turquoise and nickel pink.
During our summer holiday in Cornwall we had a lovely walk along the SW coast path from Perranporth to Portreath, passing several abandoned mines and spoil heaps, great places to look for rocks and minerals.
Cornwall, Godrevy lighthouse
We started our 12 mile walk at Perranporth, south of Newquay on the north Cornwall coast. At Cligga Head, we passed an abandoned tungsten mine, where they mined wolframite, a black mineral containing tungsten, iron and manganese. There was also a lot of greisen, decomposed granite, on the first step towards becoming china clay. While the granite mass was solidifying 300 million years ago, hot gases caused it to decompose. It is now easily weathered by the rain and sea and we could see white granules of quartz everywhere under the heather. The hot gases forced the granite up through cracks in the older Devonian slate rocks and caused minerals to dissolve and reform in seams. The ends of mine shafts can be seen in the cliff face, which is stained red with iron oxide.
Rocks and pebbles found on our walk. Bottom right, altered granite; the large white crystals are feldspar. Bottom left, turquoise rock stained with copper. Middle left, red rock stained with iron oxide. The black rocks and pebbles are Devonian slate (400 million years old), with white veins of quartz running through them.
Rocks from the SW coast path
At St Agnes head, there is a granite outcrop which has been weathered into a clay and sand deposit where clay is dug for the Leach Pottery. Further along the coast path is the remains of an old tin mine, Wheal Coates, where the chimney of the Victorian pump engine house still stands at the top of the cliffs. Tin ore was found in seams below sea level, and so water had to be pumped from the mine shafts. We tried throwing a rock down one of the old mine shafts and it took a very long time to hit the bottom.
We also had a lovely visit to the Leach Pottery in St Ives, where we were shown round the working studio by trainee potter Lexie Macleod. The potters use a mix of St. Agnes clay, ball clay from Devon and iron oxide to give the dark, toasted colour they want. They use several glazes, a green ash glaze, black tenmoku, a dolomite white and a white/orange shino. They often throw a hundred mugs in a day, which then take several more days to finish and add handles. Their outlets include Seasalt, who fund an apprentice, and David Mellor in London and Sheffield.
I am excited that my next book Science for Potters is coming out soon. The cover shows a pair of Atomic Bottles by Kate Malone, covered with atoms and crystals. The book will cover chemistry, geology, clay, crystals, the science behind colour and glazes. For anyone who would like to know a bit more about the science of pottery.
Science for Potters
Have a look at an excerpt on silica phases and eutectics: Bloomfield_May17CM
Originally published in May 2017 issue of Ceramics Monthly, pages 80-81. http://www.ceramicsmonthly.org . Copyright, The American Ceramic Society. Reprinted with permission.
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.
Copper oxide and chrysocolla (copper silicate mineral)
Cobalt oxide and skutterudite (cobalt arsenide)
Iron oxide and haematite.
Chromium oxide and chrome diopside (silicate pyroxene mineral)
Rutile in pink and yellow glazes and rutile crystals.
Monazite sand containing the rare earth oxides neodymium, praseodymium and erbium.
Nickel oxide and annabergite (nickel arsenate mineral). Pot by Avril Farley.
Vanadium pentoxide and vanadinite (lead vanadate chloride crystals)
Manganese dioxide and rhodonite (manganese silicate mineral)
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.
Quartz weighed on a triple beam balanceChina clayBorax frit
Opacifiers can be added to make the glaze opaque. Tin oxide is the most effective opacifier but zirconium silicate is less expensive.
Tin oxide
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.
Quartz
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.
Granite: feldspar (white), quartz (grey) and mica (black)
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.
Clay, dried and powdered
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.
