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Functional ware at low fire! Don't dismiss it just yet.

These were only fired at cone 04, but they are durable enough to last some time with normal use. The insides have a transparent glaze (Spectrum 700), it is leadless and completely safe. These are great insulators, they keep coffee warm longer than porcelain or stoneware. The feet are glazed so they are fine for the dish washer. They are super-light, the body is made from ball clay and talc and throws really well. These shrink very little on drying (in this case less than 2% compared to porcelains which can be 8% or more). These are inexpensive to fire, only four hours to cone 04. They withstand impacts better than you think (a thin porcelain mug propagates cracks and can shatter). Colour, glorious color! These are Spectrum Opaque low fire glazes, dozens of wild colors are available.

Context: Which is better for functional ware? Cone 04? Cone 10 reduction?, Low Temperature Glaze Recipes

Wednesday 25th November 2020

A terra cotta clay fired from cone 06 (bottom) to 4

Fired bars of a terra cotta clay fired from cone 06 (bottom) to 4

Terra cotta bodies are more volatile in the kiln than stonewares. They mature rapidly over a narrower range of temperatures, that process is accompanied by dramatic changes in fired color, density and fired strength. These bars are fired (bottom to top) at cone 06, 04, 03, 02, 2 and 4. This is Plainsman BGP, cone 02 finds it at maximum density (and fired shrinkage). At cone 06 (1830F/1000C) it is porous and shrinks very little. But as it approaches and passes cone 03 (1950F/1070C) the color deepens and then moves toward brown at cone 02 (where it reaches maximum density). However past cone 02 it becomes unstable, beginning to melt (as indicated by negative shrinkage). This is typical of most terra cotta clay materials.

Context: Why this transparently-glazed terra cotta is better at cone 03 than 04, Maturity, Terra cotta, Cone 1

Wednesday 25th November 2020

Why this transparently-glazed terra cotta is better at cone 03 than 04

Two terra cotta mugs, the glaze on one is more glossy and the body color is richer

Terra cotta bodies are typically fired between cone 06 and 04. That being said, many, like this Plainsman L215, develop richer color at cone 03 and fire much stronger. Glazes, of course, melt better and micro-bubbles pass through easier at cone 03. But this happens only if the body has not begun to decompose (and therefore generate a lot more gases of decomposition). Notice that crazing is beginning on the one of the left. Apparently the improved body:glaze interface and the development of better vitrification reduces the problem. Cone 03 is somewhat of a sweet-spot for this specific body, however firing higher begins decomposition processes that generate gases that disrupt the surface. Needless to say, accurate firing is needed to fire at cone 03 for ongoing success (because cone 02 is too high for this body, glaze will blister). Do you know what terra cotta actually is, if not click the link to learn more (this is a big topic).

Context: A terra cotta clay fired from cone 06 (bottom) to 4, G1916Q transparent on terra cotta body at cone 06, 05, 03, Terra cotta, Vitrification

Tuesday 24th November 2020

When it makes total sense to fire hotter than cone 04

Two mugs, the one fired at cone 03 is better.

The body is Plainsman L215 (bisque fired at cone 06). These commercial underglazes are not temperature-specific, they work the same across a wide range of temperatures. But the glaze is G2931K, it was designed for cone 03 and does not melt as well at cone 04 (for example, it goes milky if applied too thick), it was for use on Zero3 stoneware and Zero3 porcelain. This body is, however, a standard porous earthenware. While it is normally fired at cone 06-04, it tolerates cone 03. Actually, it benefits from cone 03, firing stronger and a deeper red color. So, since this glaze and body are both better at cone 03, it makes total sense to fire pieces like that at that temperature.

Context: G2931K, Terra cotta

Tuesday 24th November 2020

Terra cotta vs. low fire red stoneware

These terra cotta mugs are fired at cone 03. Although the glaze on the left one is melted well the terra cotta itself has a porosity of more than 10%. The mug on the right is a finer grained terra cotta with added frit to make it vitrify. It is thus dramatically stronger and more durable, rivalling high temperature stoneware. Neither of the glazes are crazed, but the glaze on the right is much more firmly attached (has a better interface with the glaze) and therefore resistant to future crazing. Does the mug on the left have an advantage? Yes. Although both can withstand hot coffee being poured in, the one on the left can withstand more dramatic thermal shocks without the piece itself cracking.

Context: Terra cotta

Tuesday 24th November 2020

A bucket of glaze smells totally rank! What to do?

A bucket of glaze slurry that smells really bad

In some places and climates this is more of a problem than in others. It is often not something that can just be ignored because the rheology of the slurry will likely be affected. Is there a magic chemical one can dump in to fix it? Not really. The subject of micro organisms in glaze slurries can be as complicated as you want to make it. This is because there are just so many different things that could be causing the stink. And there is no one chemical that treats them all. Even if there was, it's use would be focussed on prevention rather than fixing a problem. And, it would being its own issues, hazards and specific procedures. There are some simple things to know about dealing with micro organisms in glazes that should enable to you keep relatively free of this issue.

Context: Micro Organisms

Tuesday 24th November 2020

Our base glazes plus opacifiers on a dark burning body at cone 6

Red burning clay tiles with opacified versions of G2926B, G2934, G1214Z

The body is Plainsman M390. These are commonly-used base glazes. The top one is an MgO matte, next down is a calcium matte. They react very differently to these additions. Notice also the difference when titanium dioxide is applied thickly. Tin oxide fires whiter than zircon (e.g. Zircopax). Each opacifier has issues. Tin is expensive. Titanium is difficult to mix into the slurry (screening required), not as white or opaque, variations in thickness produce more difference in results and it can turn blue. Zircon is more likely to cutlery mark, twice as much is required and it amplifies the color of any iron present.

Context: G1214Z, G2934Y, G2934, An incredible M390-compatible cone 6 red-burning casting body, Opacifier

Thursday 19th November 2020

To gauge strength: Break pieces often. Pay attention to what happens.

A broken slip cast cone 6 mug

This is M340 casting, L3798H. The handle was just glued on with slip. I had questions about this new body. Are my glazes under compression? How well is the handle stuck on? Because I have broken so many pieces I know how they break. It is a good sign when there are no fractures along the handle joins. And when the handle is the last to go. And when the item breaks into large pieces rather than shattering into small ones. Try doing this on some of your pieces and you might be surprised. Either by how strong they are. Or by how easily they break.

Context: Fired Strength, Glaze Compression

Saturday 14th November 2020

How is it possible for the same body to work well at both cone 04 and 6!

The same clay body fits glazes at low temperature and is a stoneware at medium fire. How?

Plainsman 3D! White cone 04 bodies are not vitreous and strong and neither is this. But it is plastic, smooth and fits common low fire glazes. How? 15% Nepheline Syenite (also 50% Plainsman 3D, 35% ball clay and 3% bentonite). The unmelted nepheline particles impose their higher thermal expansion on the fired ceramic. Spectrum 700 clear glaze does not craze and does not permit the entry of water (the mug is glazed across the bottom and fired on a stilt). The mug on the right is made from the same clay, it has been fired ten cones higher, cone 6! Here the nepheline is acting as a flux, producing a dense and very strong stoneware (with G2926B, GA6-B glazes). This is incredible! One note: This cannot be deflocculated and used for casting, soluble salts in the 3D gel the slurry.

Context: Nepheline Syenite, 3D Clay, Low Fire White Talc Casting Body Recipe

Friday 30th October 2020

A novel way to test glaze compression and glaze fit

Two coffee mugs, one cracked, the other shattered

These are made from L4005D red cone 6 stoneware. Both are cast and thin-walled (half of what a thrown piece would be). They were glazed only on the inside to encourage cracking/splitting if the glaze is under excessive compression (that is, the thermal expansion of the glaze is significantly less than that of the body). And that is what happened here. The piece on the left cracked after a couple of taps with a hammer. Notice how the crack has opened. The piece is "spring-loaded" (press it together and it reopens on release). The glaze is GA6-B. The piece on the right is glazed with G1214Z. It spontaneously blew in half, with a loud crack, a few 5 hours after exit from the kiln. On further taps with a hammer these pieces shattered into dozens of smaller ones! The white glaze is certainly under too much compression. Obviously, neither is under any danger of crazing. Is the compression too great on the dark glaze? It did not shatter the way the white one did on further taps. And, another thicker-walled piece exiting the same kiln was glazed inside and out with that glaze. It was very strong. The lesson: Glaze compression, if not too much, is good for ware strength - but pieces must be glazed both outside and inside. And, thin ware like this, is good for testing that compression.

Context: An incredible M390-compatible cone 6 red-burning casting body, 2019 Jiggering-Casting Project, Glaze Compression

Thursday 29th October 2020

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