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Mix a whole bag of G2926B whiteware clear glaze

When you mix it right it will be thixotropic, that is, it will gel slightly and hold itself on the ware after dipping. This state can only be achieved if there is enough water for the epsom salts to do their magic. The watery nature of the slurry is nice for measuring specific gravity using a hydrometer (normally they don't float freely enough if the slurry is creamy). We normally recommend a specific gravity of 1.44 for this glaze, but in this case it seemed watery enough at 1.46. On use it will become clear if 1.46 is OK. How? It will go on the ware too thick. If that happens just add water to 1.44 and add more epson salts to gel it back up.

Wednesday 20th February 2019

Three visual glaze mechanisms make this piece unique

Three visual glaze mechanisms make this piece unique

"Mechanisms" are specifics about the glaze application or preparation process, the materials, the chemistry or firing schedule that produce a specific visual effect. This is fired at cone 10R. It is made from a buff stoneware, Plainsman H550, and has L3954N black engobe on the inside and part way down the outside. The transparent glaze on the inside gives the black a deep vibrant effect. The outside glaze is G2571A with 3.5% rutile and 10% zircopax added (the latter imparts opacity and the former produces the variegated surface). The powerful color of the black engobe wants to get through but it is only able to do so where the glaze layer is thinner (producing the varied shades of brown with differing thicknesses of glaze that occur because of the presence of the incised design).

Saturday 16th February 2019

Possible to grind your own ceramic grade rutile?

Possible to grind your own ceramic grade rutile?

Yes, the granular and powdered grades are the same material. But grinding it is very difficult. Commercial ceramic grade powder is minus 325 mesh, the companies doing this obviously have very good grinding equipment. They also have patience because even in this efficient porcelain ball mill, 90 minutes was only enough to get 50% to minus 325 mesh! The color of the powder is a good indication of its quality, the finer the grind the lighter will be the tan coloration.

Wednesday 13th February 2019

More problems measuring glaze specific gravity using a hydrometer

More problems measuring glaze specific gravity using a hydrometer

First, the hydrometer is long, the only container I have is this graduated cylinder. I had to fill it just the right level so it reads near the top. OK, fine. But the hydrometer needs to bob up and down to find home. However this glaze has a creamy consistency, that prevents free movement. OK, I will carefully help it find home by pushing it down a little. But then it doesn’t want to bob back up! Ok, I’ll pull it up and push it down and put it where I think it should float. Not great. Next problem: The glaze is opaque, I can’t see the reading. Yikes! A better way would be to throw out the hydrometer and just tare the empty cylinder on a scale, fill it to 100 and read the SG as the weight/100. If this glaze was free-flowing and watery it would be a different story, the hydrometer would be useable.

Tuesday 12th February 2019

Same glaze/body. One fired flawless, the other dimpled, pinholes. Why?

Same glaze/body. One fired flawless, the other dimpled, pinholes. Why?

The difference is a slow-cool firing. Both mugs are Plainsman M340 and have a black engobe inside and partway down on the outside. Both were dip-glazed with the GA6-B amber transparent and fired to cone 6. The one on the right was fired using the PLC6DS drop-and-hold schedule. That eliminated any blisters, but some pinholes remained. The one on the left was fired using the C6DHSC slow-cool schedule. That differs in one way: It cools at 150F/hr from 2100F to 1400F (as opposed to a free-fall). It is amazing how much this improves the brilliance and surface quality (not fully indicated by this photo, the mug on the left is much better).

Wednesday 6th February 2019

Do ceramic material powders go bad?

Do ceramic material powders go bad?

Many minerals are just ground up rocks, they were in the ground for millions of years (e.g. kaolin, feldspars, ball clays, bentonite, calcium carbonate, dolomite, talc, kyanite, wollastonite, etc), so the powders should last millions of years as well. Some are powderized man-made glasses and sintered solids, these are very stable (e.g. frits, stains). Other man-made materials are less stable and can hydrate or oxidize (e.g. carbonate colors, plaster), keep them sealed containers. Some materials are organic (e.g. Gum Arabic) and they can go bad in damp conditions, so keep them in a sealed container also.

Wednesday 6th February 2019

Cracking casseroles. Why?

Cracking casseroles. Why?

The cracks happened on heat up (since they have opened up wide). A combination of issues contribute. The kiln shelves heat-sink the wide flat bottoms, vessel walls are thick, there is some unevenness of wall thickness and only a 30-minute hold at 220F to remove glaze water from the bisque (that could have left dampness in thicker sections); these combined to produce temperature gradients within the piece. The firing schedule rose rapidly from 250-2100F (400F/hr) amplifying these gradients as it climbed. At quartz inversion these gradients produced a wave of volumetric change moving through the bisqued piece and it initiated a crack where different thickness met at a sharp contour, the bottom corner.

Monday 4th February 2019

Vitrification can be obvious by simple visual inspection

Vitrification can be obvious by simple visual inspection

The unglazed surface of the left piece has a sheen, it is a product of glass development during firing to cone 6. That body is a 50:50 mix of a cone 8 stoneware and a low fire earthenware red (a material that would normally be melted by this temperature). Together they produce this dense, almost zero-porosity ceramic. The unglazed surface on the right looks more like plaster, and it is absorbent, about 5% porosity. It is a mix of the same stoneware but with 50% ball clay. The refractory ball clay assures that the stoneware, which was already inadequately vitreous, is even more so. As you can imagine, the left piece is far stronger.

Monday 4th February 2019

The matteness this glaze develops is dependant on the cooling rate

The matteness this glaze develops is dependant on the cooling rate

This is the G2934Y matte cone 6 recipe with a red stain (Mason 6021). The one on the left was fired using the C6DHSC slow-cool schedule. The one on the right was fired using the drop-and-soak PLC6DS schedule. The only difference in the two schedules is what happens after 2100F on the way down (the slow-cool drops at 150F/hr and the other free-falls. For this glaze, the fast cool is much better, producing a silky pleasant surface rather than a dry matte.

Monday 4th February 2019

200 Shimpo wheels just arrived, ready to be certified

200 Shimpo wheels just arrived, ready to be certified

January 2019. Another shipment of wheels and pugmills from Nidec-Shimpo. Although a large company, making drive mechanisms for many types of heavy equipment, they apply their technology to potter's wheels as a matter of pride in a country that reveres pottery in its culture. Every box has been opened to reveal the existing serial number. A certified inspector will check every one of them and affix another sticker to assure they meet CSA Code SPE-1000 for electrical safety. This approval enables the sale of the equipment to public institutions. And it assures your that the equipment meets CSA electrical standards and is safe to use.

Tuesday 22nd January 2019

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