Two stains. 4 colors. Will the guilty oxide please step...

Three cone 6 commer-cial bottled glazes have been layered. The mug was filled with lemon juice over night. The white areas on the blue and rust areas

on the brown have leached! Why? Glazes need high melt fluidi-ty to produce reactive surfaces like this. While such are normal-ly subject to leaching, the manufacturers were able to tune the chemistry of each to make them resistant. But ...

The glaze on the right is crawling at the inside corner. Multi-ple factors contribute. The angle between the wall and base is sharper. A thicker layer of glaze has collected there (the thicker it is the more power it has to impose a crack as it shrinks during drying). It also shrinks more during drying because it has a higher water content. But the leading cause: Its high ...

We are looking at two pairs of samples, they demonstrate why knowing about glaze chemistry can be so important. Both pairs are the same glazes: G2934 cone 6

matte and G2916F cone 6 glossy. The left pair has 5% maroon stain added, the right pair 5% purple stain. The red and purple develop correctly in the glossy but not the matte. Why? The Mason Colorworks reference guide has the same precaution for both stains: the host glaze must be zincless and have 6.7-8.4% CaO (this is a little unclear, it is actually ...

These are thermal expansion curves for body, engobe and glaze (from a dilatometer, a device that measures it against increas-ing temperature). The upper line is the body. The center line is the engobe. The lower line is the glaze. The ceramic tile indus-try is very conscious, not only of glaze-fit but also engobe-fit. Engobes (slips) are employed to cover brown or red burning bodies so they glaze like a porcelain. Typically technicians tune the formulation of the engobe to have ...

Two stains. 4 colors. Will the guilty oxide please step forward!

Potters can learn from how glazes are fit on ceramic tile

Two glazes. One crawls, the other does not. Why?

This leaching mug needs a liner glaze. Seriously!

To see the rest scan the QRCode or visit http://digi-talfire.com/techtips/1030




This mug is pinging loudly and literally self-destructing in front of my eyes! Why? The glaze is under so much compression (the inside is pushing out-ward, the out-side inward). Spiral cracks

are developing all the way up the side. Small razor-sharp flakes are shivering off convex contours. Why? I accidentally fired a low-temperate talc body at cone 6 (the glaze is the Alberta Slip base cone 6 glossy). The clay body is not overly mature, but it just has an extremely high thermal expansion (talc is added to increase the expansion to fit low fire ...

Slips and engobes are fool-proof, right? Just mix the recipe you found on the internet, or that someone else recommends, and you are good to go. Wrong! Low fire slips need to be compatible with the body in two principle ways: drying and firing. Terra cotta bodies have low shrink-age at cone 06-04 (but high at cone 02). The percentage of frit in the engobe determines its firing shrinkage at each of those temperatures. Too much and the engobe is stretched on, too little and it is under

compression. The lower the frit the less the glass-bonding with the body and the more chance of flaking if they ...

It seems logical (and convenient) to just say that the kiln does not care what materials source the oxides in a glaze melt. Li2O, CaO, Al2O3, SiO2 are oxides (there are about ten common

ones). The kiln just melts everything and constructs the glaze from the ones available. Right? Wrong! Things get more com-plicated when frits are introduced. Frits are man-made glasses, they melt much more readily than raw materials like feldspar. Raw materials are often crystalline. Crystals put up a fuss when asked to melt, often holding on as long as they can ...

I used a binder to form 10 gram balls and fired them at cone 08 (1700F). Frits melt really well, they do not gas and they have chem-

istries we cannot get from raw materials (similar ones to these are sold by other manufacturers). These contain boron (B2O3), it is magic, a low expansion super-melter. Frit 3124 (glossy) and 3195 (silky matte) are balanced-chemistry bases (just add 10-15% kaolin for a cone 04 glaze, or more silica+kaolin to go higher). Consider Frit 3110 a man-made ...

Glazes of the same chemistry: The fritted one melts better

Do you know the purpose of these common Ferro frits?

Bi-Clay strips test compatibility between engobe and body

How to make a ceramic time-bomb





This reduction celadon is crazing. Why? High feldspar. Feldspar sup-plies the oxides K2O and Na2O, they contribute to brilliant gloss and great color (at all temperatures) but the price is very high thermal expansion. Any glaze having 40% or more feldspar should turn on a red light! Thousands of recipes being traded

online are high-feldspar, some more than 50%! There are ways to tolerate the high expansion of KNaO, but the vast majority are crazing on all but high quartz bodies. Crazing is a plague for potters. Ware strength suffers dramatically, pieces leak, the glaze harbours bacteria, crazing invites customers to ...

G2934 is a popular matte for cone 6 (far left). It is not matte because it is not melting enough or is covered with micro-crys-tals, it is an MgO matte (a mechanism produces a more pleasant surface that cutlery marks and stains less). But what if it is too matte for you? This recipe requires accurate firings, did your kiln really go to cone 6? Proven by a firing cone? If it did, then we need plan B: Add some glossy to shine it up a bit. I fired these ten-gram balls of glaze to cone 6 on porcelain tiles, they melted down into nice buttons that display the surface well. Top row proceeding right: 10%, 20%, 30%, 40% G2926B ...

Worthington Clear is a popu-lar low fire transparent glaze recipe. It has 55% Gerstley Borate plus 30% kaolin (Ger-stley Borate melts at a very low temperature because it sources lots of boron). GB is also very plastic, like a clay. I have thrown a pot from this recipe! This ex-plains why high Gerstley Borate glazes often dry so slowly and shrink and crack during drying. When recipes also contain a

plastic clay the shirinkage is even worse. GB is also slightly sol-uble, over time it gels glaze slurries. Countless potters struggle with Gerstley Borate recipes. How could we fix this one? First, substitute all or part of the raw kaolin for calcined kaolin (using 10% less because it has ...

Custer feldspar and Nepheline Syenite. The coverage is per-fectly even on both. No drips. Yet no clay is present. The secret? Epsom salts. I slurried the two pow-ders in water until the flow

was like heavy cream. I added more water to thin and started adding the epsom salts. After only a pinch or two they both gelled. Then I added more water and more epsom salts until they thickened again and gelled even better. They both applied beautifully to these porcelains. The gelled ...

Can you actually throw a Gerstley Borate glaze? Yes!

Feldspar applied as a glaze? Yes! The way I did it will change how you glaze.

Tune your matte glaze to the degree of matteness you want

A down side of high feldspar glazes: Crazing!





Recipes show us the materials in a glaze but formulas list oxide mol-ecules and their compar-ative quantities. Oxides construct the fired glass. The kiln de-constructs ceramic materials to get the oxides, discards the carbon, sulfur, etc. and builds the glass from the rest. You can view glazes as recipes-of-materials or as formulas-of-oxides. Why

use formulas? Because there is a direct relationship between the properties a fired glaze has (e.g. melting range, gloss, thermal expansion, hardness, durability, color response, etc) and the oxides it contains (links between firing and recipe are ...

These two glazes are both brilliant glass-like su-per-transparents. But on this high-iron stoneware only one is working. Why? G3806C (on the outside of the piece on the left) melts more, it is fluid and much more runny. This melt fluidity gives it the capacity to pass the micro-bubbles generated by the body during firing. G2926B (right) works great on porcelain but it cannot clear the clouds of micro-bubbles

coming out of this body. Even the glassy smooth surface has been affected. The moral: You need two ...

Both mugs have the same cone 6 oxi-dation high-iron (9%), high-boron, glossy glaze. Iron silicate crystals have

completely invaded the surface of the one on the right, turning the near-black glossy into a yellowy matte. Why? Three things. It was slow-cooled and the other free-fall-cooled (firings done in the same kiln). The glaze has a fluid melt (it runs) and its percentage of iron is high enough that it could precipitate out from solution in the melt (given the time). Susceptible glazes have a temperature at which crystals form the best and that temperature can be hundreds of degrees down from the firing cone (or higher if precipitation is ...

The key is avoidance of methods that result in one part of the piece being stiffer at any stage of drying (not vinegar in the water, compressing the bottoms, etc.). Throw mugs with walls of even thick-ness. Put them on a plaster bat (it dewa-

ters the base). Make the handles a while after you have made the mugs (they stiffen quicker). Apply them as soon as the rims are stiff enough to maintain shape (in my climate, two hours). Use a join method that enables application of lots of pressure (better than scoring). Use only enough slip (of thick cream consistency) to make the join (no excessive ...

Same high-iron glaze. One crystallizes and the other does not. Why?

Sixteen kinds of clay. No mugs have cracked in drying. Why?

Two transparent glazes on the same dark burning clay. Why different?

A limit or target glaze formula. What does this mean?





Two stains. 4 colors. Will the guilty oxide please step...

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