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Think the idea of mixing your own glazes is dead? Nope!

These are two pallets (of three) that went on a semi-trailer load to a Plainsman Clays store in Edmonton this week. They are packed with hundreds of bags of powders used to mix glazes. More and more orders for raw ceramic materials are coming in all the time. Maybe you are using lots of bottled glazes but for a cover or a liner glaze it is better to mix your own. And cheaper! And there are lots of recipes and premixed powders here to do it. One of the big advantages is that when you dip ware into a properly mixed slurry it goes on perfectly even, does not run and dries on the bisque in seconds. No bottled glaze can do that.

Context: Where Do I Start?, Glaze Mixing, Dipping Glaze, Base-Coat Dipping Glaze, Plainsman Clays

Tuesday 11th August 2020

Fast cooling vs. slow cooling Alberta Slip GA6-A transparent base

These two mugs have the Alberta Slip base cone 6 GA6-A glaze on the inside. The left one is cooled normally (kiln off at cone 6 after soak). For the mug on the right the kiln has been soaked for half an hour at 1800F on the way down. This was done to develop the rutile blue glaze on the outside, but during this period crystallization occurred on the inside. If you need to cool slow (for the Alberta Slip rutile blue) but would like the liner glaze to be transparent, add 0.5-1% tin oxide to the GA6-A to impede crystal growth.

Context: GA6-A, Crystallization

Thursday 6th August 2020

Mason stains in G2916F cone 6 clear base

These are Mason stains added to cone 6 G2916F clear liner base glaze. Notice that all of these stains develop the correct colors with this base (except for manganese alumina pink 6020). However caution is required with inclusion stains (like #6021), if they are rated to cone 8 they may already begin bubbling at cone 6 is some host glazes.

Context: G2916F, Stains Mason, Ceramic Stain, Base Glaze, Dipping Glaze

Thursday 6th August 2020

M340 Transparent base glaze with Mason stains

M340 Transparent Liner glaze fired at slightly lower than cone 6. Using these modest stain amounts the degree of melting of the glaze is not overly affected (these were balls, they flattened during firing). However as a glaze layer on a body, many of these will not be as dark as you see here.

Context: G2916F

Thursday 6th August 2020

The handle did not break. Despite repeated whacks with a hammer. Why?

A broken mug, lying in pieces, but the handle is intact

This body vitrifies to incredible strength on its own. But applying a glaze complicates that. The inside glaze, G2926B, is under thermal expansion compression. The outside glaze (which I neglected to sieve) only covers about half of the surface (the rest is the bare body), and it is under less compression by the body. Visibly, the piece appeared fine, but internally the compressive stress was seeking relief. Before describing what happened when I hit this with a hammer, I will tell you what happened when I broke another mug of this same type, glazed inside and out with G2926B clear: It required a dozen whacks! It sounded like I has hitting a piece of steel! But the lower section of this broke off with the first whack! Subsequent hits broke off other pieces. But notice the handle, which is entirely glazed, has not broken. Even though the wall of the mug has entirely separated inside of it. This is a testament to how much the glaze strengthens the handle (by its full coverage) and how much it weakens the body (by partial coverage).

Thursday 6th August 2020

A matte and a glossy liner glaze

Left: Ravenscrag G2928C matte liner glaze on inside of mug. Right: A clear glossy. The matte needs to be soaked in the kiln long enough to make sure it develops a functional surface, especially on the bottom. Mattes are not always the best choice for food surfaces, but you can do it if you blend in enough glossy glaze to make it smooth enough not to cutlery mark.

Context: G2928C, How to Liner-Glaze a Mug, Food Safe, Dipping Glaze

Thursday 6th August 2020

Cone 6 copper glaze works because of the fluid-melting base recipe

This is not just a typical transparent cone 6 glaze with 2% copper carbonate added (and 2.5% tin oxide). Knowing what is different about this clear base, its trade-offs and how it was developed are important. The porcelains are Plainsman P300 and M370. The liner glaze is G2926B, it is a gloss but has a much lower melt fluidity than the outer glaze, G3806C (as a functional transparent its main job is to fit the body and be hard and durable). But in order for that outer glaze to accommodate the copper and still be super glossy it must have a much higher melt fluidity. It was tricky to develop since that fluidity comes with high sodium and lower silica, that raises the thermal expansion and moves it toward crazing.

Context: G3806C, Fluid Melt Glazes

Thursday 6th August 2020

Sterile white vs. pure Ravenscrag Slip as a liner glaze at cone 10R

This picture does not fully convey how much better the Ravenscrag is as a liner glaze (vs. G1947U). It has depth and looks much richer. It course, it could be opacified somewhat to be whiter and would still retain the surface quality (as long is it is not too opaque). The body is Plainsman H450. The outside glaze is pure Alberta Slip.

Context: G1947U, GR10-A, Roasting Ravenscrag Slip instead of calcining

Thursday 6th August 2020

Maroon and white mug before and after firing: What a difference!

The outer glaze is Ravenscrag GR6-E Raspberry, the bright maroon color is a product of the surprising interaction between the 0.5% chrome oxide and 7.5% tin oxide present. That small amount of chrome is only enough to give the raw powder a slight greenish hue, hardly different than the clear liner glaze. While this color mechanism appears to be effective, it is delicate. A maroon stain is actually a better choice. It would fire more consistent would be less hazardous to use. And the raw glaze will be the same color as the fired one!

Context: GR6-E, Colorant, Ceramic Stain

Thursday 6th August 2020

Iron oxide as a debubbling agent at low temperature

One glaze is transparent, the other milky because it is filled with micro bubbles.

These terra cotta clays were bisque fired at cone 04 and glaze fired to 04 using the 04DSDH schedule. The glaze is G1916Q, an expansion-adjustable cone 04 clear. That schedule alone is often enough to get transparent, defect free glazes in many situations. But not in this case. The solution was to add a fining agent. In this case we added 2% red iron oxide (to the top glaze). The particles of iron floating in the melt acted as a congregating points for bubbles, helping them to escape. And we got a bonus: a more interesting aesthetic. We did further tests and determined that a 1% addition also worked, but not as well. And screening out the larger particles slightly degraded the fining performance. So we have settled on 2% iron and screening the glazes to 100 mesh. Although iron works here, it will not always do so in other situations. And, other fining agent agents we have used at cone 6 do not work in this situation (e.g. 2% Zircopax, Alumina).

Context: Glaze Bubbles, Terra cotta, Transparent Glazes, Drop-and-Soak Firing

Thursday 6th August 2020

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