Techno Tip Embarrassed in the Pink: Stopping Chrome Flashing Close your eyes and imagine a nice pink pastel glaze. Now imagine that you wanted a nice white instead! During the ACerS convention in St. Lowais, | attended an eye- opening presentation by Stan Sulewski of Pfalzgraff (they are a well known porce- lain tableware manufacturer in the US). After heanng what he said I came to better appreciate the synergy between chemistry and physical properties involved with glaze opacity and color. Potters absorbed in re- active and artware glazes often forget how difficult it can be to make a proper white semi-gloss food-safe glace. Engincers at the company faced a.dileme€sa: * The tin opacified glaze flashes pink because chrome used in the darker coloured glazed ware volatilizes and reacts with the tin to form chrome tin pink hues in the white. + The zircon opacified glaze had excessive metal marking, Re- fractory and angular zircon par- ticles protrude from the surface when their population is too high (even when particle: size is. very fine). ‘The obvious solutions of dedicating a kiln to non-chrome-bearing ware or eliminat- ing chrome containing glazes were not feasible, Thus the objective was clear: Adjust the recipe of the tin glaze to have low metal marking and white colour with- out pink flashing. The first and most obvious approach of simply blending tin and zircon would ad- dress the marking problem but the pink colours of course remained because tin is 80 sensitive to chrome. Impossible as it may seem, they actually found an answer using ceramic chemistry. If you have ever worked with chrome tin stains, you likely know that unless the chemistry of the host glaze is night the colour does not develop. Getting the col- our fo work cant be a real challenge but in this case they actually wanted to sabotage it! Among wamings on stain manufactur- ers chrome-tin data sheets are mentions of the detrimental effects of zinc, raw alumina, magnesia and a lack of calcia. They rea- soned that it should be possible to solve this problem by making the host glaze chemistry hostile to the development of chrome tin pinks, And that is what they did. Zinc, the most obvious choice, did kill the pink but it also imparted a yellow brown colour that would not do. The presence of adequate CaO is critical to the develop- ment of pink and MgO is detrimenial. While both are fluxes, a complete replace- ment was not practical. Glazes tolerate and usually benefit from relatively large amounts of CaQ), but complete replace- ment with MeO or SrO (or even a mix} produces much different surfaces and less active melting. The critical factor, as implied above, is that if calcium is mot present in a threshold minimum amount chrome tin pink colours can be completely absent, Thus the answer tumed out to be a compromise: a MgOV $rO mix (with more MgO) replacing much of the CaQ. This preserved the surface character and killed the pink. However the white colour was compro- mised just a litthe oo a final adjustment was done: a smal] amount of blue stain was added to brighten the white. There you have it, ceramic chemistry to the rescue again! However the story is not quite over, they still need to adjust things to better match the thermal expansion of the new glaze with the old. Tony Hansen Techno Tip iy generously supported by Pladnsean Claeys Limited in Medicine Hat, Alberta and its afilimes, Greenbarn Potters Supply in Surrey and Vancouver [star Pottery Wirrehowse in Marualeia, Potters Guild of British Columbia Newsletter Raku U Granville Island, Canada Day 9:00AM - 6:00PM Volunteers are needed to help out with the Raku U. This is one of the biggest fundraisers for the Guild, and is enjoyed by hundreds of people. If you can help out with a few hours or even better, all day, please call Maggi at 6(4.929,3206, Lunch and soft drinks will be provided. Important Notice Deadline: July/August issue Submissions for the last newsletter before summer is Wednesday, June 5. Table of Contents Rie Suzuki Techno Tip Raku U Volunteers Made of Clay Wrap-up President's AGM Report Gallery AGM Report Place Settings Exhibition Laurie Rolland Exhibition Call for Applications Mary Allesia Scholarship Workshops Peter Isley Workshop Gordon Hutchens Wshp Made of Clay Presenters ClayLines Wanted 12 Classified IZ Important Contacts 12 i i i => July/Aug Newsletter Deadline Wednesday June 5 Send to Guild office, see page 12, or email editor: June 2002