Что такое глазурь?

Объяснение доктора Массачусетского института технологии  Кэрол Марианс. По сообщению Мела Джакобсона в Клэйарт.

a great explanation of/ what is glaze? carol marians/2 phd’s from MIT chemistry and physics. one of the leading experts in silica/quartz in the world.  and a friend of potters.
(from the small book on down firing, and the kiln book.

carol has been very generous in sharing vital information about long down firing. (AND OF COURSE, WHAT IS GLAZE?) she makes her own pots, fires them cone 6 electric with her own glazes, with very long down firing protocols.  (twenty hours)

(GLAZE AS ROCK)

Potters make their glazes from ground-up rocks.

Most of us get these rocks pre-sorted, cleaned, segregated, and ground into powder. It wasn’t always so, as books about the making of glazes from primitive materials show. Even today glazes can be, and are, made by pulverizing rock, mixing it with some material so it can be spread on the pot, and firing it! A viable glaze results! Of course, not ALL rocks lend themselves to this treatment, but plenty do.

Does the final glaze look like the rock it came from? No. If you pick up a rock and examine it under strong light, possibly with a magnifying glass, you will see speckles, streaks, bands, maybe variegated bits of different colors. Ground up, melted, re-solidified rock is not at all like its ‘parent’.

The difference between the rock and the glaze made from it is time: the interval during which the original rock cooled, and the interval during which the glaze cooled from cone whatever to room temperature!

Geophysicists know that enormous cooling time, largely under equilibrium conditions, leads to a rocky outcropping containing quartz.

(This is not the only way Nature achieves natural quartz) What matters to potters is that conditions and rate of cooling—slowly over millennia —can turn the same magma into a variety of rocks.

(The well-known exception is lava: magma spurted out from volcanoes, splashing to earth, cooling very quickly. There is no chance at crystallization, and the solidified lava forms a host of glassy rocks ranging from obsidian to pumice.) At high temperatures molecules move fast and easily, tend not to stick to each other, and can form the kind of bonds that, cooled very slowly, achieve ordered form—namely crystals. With rapid cooling the molecules get ‘rattled’, form the wrong kind of bonds—ending up as glass.

Cooled slowly enough most rocks will end as a conglomerate of many crystalline minerals.

Molten glazes never had the uniformity of a geologic magma, and cool infinitely faster, forming little crystal and much glass. Cool slowly—crystals. Cool fast—glass.

Looking now just at glazes.

What happens as a glaze melts? What happens as it cools? The paths are different but the process quite similar.

Never a uniform melt, glaze is more like a slurry of mud.

But no water, as that would have evaporated early in the firing! This slurry forms a glassy matrix in which float many liquids and solids. There are globules and bubbles of a variety of glasses, like a concoction of house paint, petroleum jelly, honey, vinegar, crankcase grease, mixing together. The heat does the mixing.

As glaze melts lots of different viscous liquids form. They don’t move far nor fast, but slowly merge. When the various solids have been incorporated into liquids, and the mixture has spread itself evenly over the pot, and cooled, we speak of a matured glaze.

Скорость охлаждения

Сью МакЛеод приводит фото «абсолютно идентичной глазури, из одного замеса, на идентичных чашках», которые были обожжены в разных печах на одну температуру. Отличие — в темпе охлаждения. В небольшой печи цикл обжига составил 8 часов, в большой — 24.

В состав глазури входит 4% оксида титана, образующего при кристаллизации розовые кристаллы. Основа глазури — зеленого цвета.  В результате цвет и фактура поверхности выглядят существенно по-разному.