Guild Diaries: Path to Pinks 

clé guild fundamentals, work study

[Ed Note: The Guild Diaries are an ongoing column looking behind the scenes at the clé Guild, our team of in-house artisans forging a new era for humble brick tile with unsurpassed levels of quality and new, elevated design narratives. It’s a chance to go deep on process and find out why we venerate brick as much as we do.]

not long after the launch of clé Guild Fundamentals glazed brick, the conversation turned to the question of what makes a fundamental? What types of surfaces, colors, tones needed to be added to the black and white surfaces that the Guild was producing as part of the initial launch?

Soon it was determined that a collection of neutrals (taupe, gray, camel, beige) would flesh out the fundamentals collection, complementing the original singles and completing the album. So began the clé Guild Work Study collection. (learn more about the collection and its inspirations.) at clé, our conviction is that pinks are the new neutrals. And thus, it inevitably followed that a selection of taupe-pinks would complete the circle.

Our first hurdle in creating these new surfaces was in simply finding means of coaxing this finicky sector of the color wheel into our glazes. As the firing temperatures rise in ceramic production, the options of colorants that will survive the heat are winnowed. The guild tiles are fired at very high temperatures, much too hot for the colors we were attempting to achieve. Historically, this palette was the province of inhospitable oxides of cadmium and uranium, fortunately no longer employed. In order to get the colors we wanted at a temperature that our clay would accept, we had to thread the needle and reformulate our glazes to mature at a slightly lower temperature that the colorants could (barely) tolerate. We conducted tests in our small electric kiln and derived surfaces that satisfied the firing temperature requirements of both clay and glaze colorants.

All seemed well with the new formulations, launch plans stirred to life, and we were set to scale the pink collection to our production gas fired kilns. But what had worked brilliantly in the electric kiln was failing miserably in the gas kilns as the previously sumptuous, silken matte surfaces were riddled with pocked blisters in the otherwise smooth glaze surface. Something was causing bubbles in the molten glaze as the minerals melted to form glass at peak temperature. Examination of the glass blisters revealed tiny iron nodules in each of the craters. The same iron inclusions that give the New California collection its wild, natural speckling were causing our pink mattes to revolt.

Something about the iron behavior in the gas kiln was causing the blisters that were not occurring in the electric kiln. We determined that in the gas fired atmosphere in the kiln, as some of the ambient oxygen was being robbed by the combustion reaction, the heat-activated iron oxide was chemically decomposing, thereby reducing to metallic iron and shedding its oxygen atoms as gas in the process. As this gas was released under the molten glaze, it formed bubbles that became frozen as the glass cooled. In the electric kiln, there was sufficient oxygen to allow the iron nodules to comfortably remain in the iron oxide form and not release gas. We needed to somehow get the glazes to perform at a temperature below which the iron oxide decomposed.

The building brick industry typically fires their bricks at or below 2040ºF for this very reason. The red color of bricks is derived from iron oxide in the clay, firing them above this temperature releases oxygen gas from the iron. This, in turn, causes the clay to foam and bloat, forming the misshapen clinker bricks loved by the arts and crafts brick masons of the early 20th century.

Our task was to try again to achieve a proper glaze melt with strong, stable glass chemistry at a temperature just lower than the iron decomposition temperature. After again reformulating our melt temperature, and adjusting silica for a slightly less viscous glass, we achieved a blister-free glaze but found the colors expressing completely differently than the original formulation. Further tests introduced new pigments, adjusted ratios and finally we had completed our trek through firing temperature, melt chemistry and atmospheric oxygen ratio to where we had started: very much the same but entirely different.