Glaze Group Meeting

14th December 2004

Di opened the meeting with her glaze test pieces using Mike's formula of crushed, ground fired ball clay with cobalt oxide addition which she had adhered to her bisque pieces.  (Mike's recipe:  whiting 30; quartz 30; china clay 30; titanium dioxide 10; zinc 2).  She had fired to a lower temperature of 1130C the result being a definite blue mottled surface with less movement and fusion than Mike's higher firing had.  Di had also brought her results using an oxide mix of 5 tin + .5 chrome in the hope of achieving a good pink.  This has been painted onto a tin glaze first with a clear glaze brushed on top (fired to 1200C).  A pleasant lavender colour was achieved.  She then made a slip using the same oxide ratio with Charles Artificial Ash Glaze over the top and was disappointed to find no colour at all.  She will continue with the experiment and increase the ratio to 10 tin + 1 chrome and add this to the actual tin glaze itself, firing temperature to remain the same.

Anne had received good results with her first use of John Britt's oil spot Tenmoku recipe featured in Mike Bailey's Oriental Glazes book: Potash Feldspar 25; Soda Feldspar 35; Ferro Frit F (3110) 5; China Clay 15; Dolomite 5; Talc 5; Flint or Quartz 8; Bentonite 2; Red Iron Oxide 6.  Applied onto an off-white body by dipping three times, firing to 1265C in her gas kiln with a soak for 30 minutes she had a pronounced attractive lusted oil spot with Tenmoku colouring. NB: Anne's only change to the recipe was by substituting Calcium Borate Frit for Firt 3110.

Rose then gave a short talk on crystalline glazing - its trials, tribulations and sometimes great results.  With two year's experience so far (comparatively short for this particular route) success in the first instance had been obtained by using one of many zinc crystalline glazes available in an electric kiln.  It is the combination of zinc and silica (zinc silicate) that mimics the formula of willemite a naturally occurring crystal.  Typical recipe could be Frit (3110) 59; zinc oxide 24.60; flint 15.90; china clay .50 with cobalt carbonate 1.  Suggested clay bodies for good crystals are porcelain and white stoneware for their relative purity with thrown rounded shapes showing off crystals particularly well.  According to the particular glaze recipe, application thickness can vary between 1mm and 4mm onto normal bisqued pots graduating thinly towards the bottom of the pot.  Because of the very fluid nature of the glaze, a catcher needs to be attached to the pot base to allow excess glaze to collect and save kiln bats.  The ramp for crystalline glazes tends to be as fast as the kiln will safely allow to peak temperature (Cone 9-12) in many instances, so that the caustic nature of the glaze does not have chance to interact with the body and perhaps contaminate the crystals.  Next rapid cooling to the crystal growing range (this is probably the most difficult part of the procedure to get right) many experiments are needed, followed by an extended soak or slowly dropping the temperature during crystal growing stage and then shutting kiln off and cooling naturally.  Books recommended are:  Peter Ilsley's Macro-crystalline Glazes - The Challenge of Crystals published by the Crowood Press and 'Crystalline Glazes' by Diane Creber published by A & C Black.

Glaze Group meetings are held at the home of Diana Carter on the second Tuesday of every other month. The next meetings will be on February 8th and 12th April 2005 at 12 noon.  Bring a contribution to a potter’s lunch and please let Charles Stileman (01794 340193) or Diana Carter (01962 777297) know if you are coming.