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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

Liquify of a pint of brushing glaze. It's easy!

Liquify of a pint of brushing glaze. It's easy!

I counter-balanced the measuring cup and weighed out 250g of water. Then I added 100g of Laguna gum solution and stirred it. I put that into the blender and added 500g of powdered glaze (you can use any glaze recipe). I started the blender on slow then increased it one-at-a-time to full speed. After less than a minute (and a little work with a spatula) it was creamy smooth. It painted evenly on the tile just like a commercial bottled glaze, drying slowly. This produces a specific gravity of 1.58 (which is pretty high) so I can add water and thin it with no issues.

Thursday 6th December 2018

G2934Y glaze with stains on P300 and Polar Ice

G2934Y glaze with stains on P300 and Polar Ice

Mugs hand-built by Tony Hansen. This base glaze is an adjustment to the original G2934 matte. It employs a frit to source the MgO instead of dolomite. The result is a glaze that melts and flows very well, yet is matte. And it is a great host for a wide range of stains, they look better in this than in a glossy base. The only color that has not worked well so far is purple.

Thursday 6th December 2018

The translucency of Polar Ice porcelain

The translucency of Polar Ice porcelain

This is Plainsman Polar Ice. Fired at cone 6 (2200F) with a transparent glaze on the inside and G2934Y yellow silky matte on the outside. This yellow glaze showcases the translucency better than any other we have seen.

Tuesday 4th December 2018

Transparent glazes often work poorly on dark stoneware bodies

Transparent glazes often work poorly on dark stoneware bodies

These are fired in cone 6 oxidation. They are all the same clay body (Plainsman M390). The center mug is clear-glazed with G2926B (and is full of bubble clouds). This dark body is exposed inside and out (the other two mugs have a white engobe inside and midway down the outside). G2926B clear glaze is an early-melter (starting around cone 02) so it is susceptible to dark-burning bodies that generate more gases of decomposition. That being said, the other two glazes here are also early melters, yet they did not bubble. Left: G2926B plus 4% iron oxide. That turns it into an amber color but the iron particles vacuum up the bubbles! Right: Alberta Slip GA6-A using Ferro Frit 3195 as the melter. It also fires as an amber-coloured glass, but on a dark body this is an asset.

Sunday 2nd December 2018

CMC Gum is magic for multi-layering, even for raw Alberta Slip

CMC Gum is magic for multi-layering, even for raw Alberta Slip

The glaze on the left is 85% of a calcine:raw Alberta Slip mix (40:60). It was on too thick so it cracked on drying (even if not too thick, if others are layered over everything will flake off). The solution? The centre piece has the same recipe but uses 85% pure raw Alberta Slip, yet it sports no cracks. How is this possible? 1% added CMC Gum (via a gum solution)! This is magic, but there is more. It is double-layered! Plus very thick strokes of a commercial brushing glaze have been applied. No cracks. CMC is the secret of dipping-glazes for multi-layering. The down side: More patience during dipping, they drip a lot and take much longer to dry.

Thursday 29th November 2018

Gum does not work in a glaze if an important ingredient is missing

Gum does not work in a glaze if an important ingredient is missing

These brush-strokes of gummed glaze are painted onto an already-fired glaze. Gummed glazes can do this, they will adhere and dry without cracking. And dry hard and resist washing off. Brush strokes hold their character. The brown glaze has 1.6 specific gravity (SG) and about 1.5% CMC gum. The white one has the same gum content but an SG of 1.5. It's brush stroke has flowed flat and it is running downward. Is it because of the lower SG? No. Commercial glazes with an SG down to 1.3 perform well also. The secret: Gum needs particle surface area to work its magic. We can get that with a bentonite addition. The dried strokes on the right were much better, that glaze adds 2% bentonite (and we raised the SG to 1.6). That made all the difference, it painted beautifully.

Friday 23rd November 2018

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

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

Custer feldspar and Nepheline Syenite. The coverage is perfectly even on both. No drips. Yet no clay is present. The secret? Epsom salts. I slurried the two powders in water until the flow was like heavy cream. I added more water to thin and started adding the epsom salts (powdered). 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 consistency prevented them settling in seconds to a hard layer on the bucket bottom. Could you do this with pure silica? Yes! The lesson: If these will suspend by gelling with epsom salts then any glaze will. You never need to tolerate settling or uneven coverage for single-layer dip-glazing again! Read the page "Thixotropy", it will change your life as a potter.

Friday 23rd November 2018

Common dipping glazes converted to jars of brushing glazes

Common dipping glazes converted to jars of brushing glazes

These are cone 6 Alberta Slip recipes that have been brushed onto the outsides of these mugs (three coats). Recipes are GA6C Rutile Blue on the outside of the left mug, GA6F Alberta Slip Oatmeal on the outside of the center mug and GA6F Oatmeal over G2926B black on the outside of the right mug). One-pint jars were made using 500g of glaze powder, 75g of Laguna CMC gum solution (equivalent to 1 gram gum per 100 glaze powder) and 280g of water. Using a good mixer you can produce a silky smooth slurry of 1.6 specific gravity, it works just like the commercial bottled glazes. Amazingly, the presence of the gum also makes it unnecessary to calcine the Alberta Slip.

Friday 23rd November 2018

Better to mix your own cover glazes for production?

Better to mix your own cover glazes for production?

Yes. In this case the entire outside and inside of the mug need an evenly applied coat of glaze. In production, it would not make sense to attempt this by painting. For these reasons: Cost, quality, convenience. The right pail has 2 gallons of G2934Y base with 10% Cerdec yellow stain: $135. Cost of jars with the same amount: Almost $300! And you have to paint them on in three coats with drying in between. The one in the pail is a true dipping glaze (unlike dipping glazes sold by glaze manufacturers that dry slowly and drip-drip-drip just like brushing ones). This one dries immediately after dipping in a perfectly even layer (if mixed according to our instructions). And a bonus: This pail can be converted to brushing or base-layering versions using CMC gum.

Friday 23rd November 2018

Calculate the total shrinkage of a porcelain hand-made tile

Calculate the total shrinkage of a porcelain hand-made tile

Plainsman Clays publish dry and fired shrinkage data for their clay bodies. Dry shrinkage is, of course, the shrinkage from wet to dry. Fired shrinkage is not, however, the total from wet to fired. Rather it is the shrinkage from dry to fired. And you cannot just add the dry and fired numbers together to get the total because the fired shrinkage value is based on the dry length, not the original (in this example, 6.25 dry shrinkage plus 6.66 fired equals 12.9 whereas the actual total shrinkage is 12.5). It is not a huge difference but this is the way to calculate it correctly if you only have drying and fired shrinkage. Thanks to Tom Hittie for deriving this for us.

Tuesday 20th November 2018

Using commercial glazes? You still need to know about specific gravity.

Using commercial glazes? You still need to know about specific gravity.

The glaze in this jar was 'goop', impossible to paint on. I did not know whether I needed to add water or try to deflocculate it (although the former is more likely and in keeping with what Laguna says on its website). I measured the specific gravity, it was 1.7, so clearly it needed water. It took 125cc to bring the specific gravity down to 1.5. However, it was still thick and dried immediately after painting on, clearly it does not contain enough gum for brushing. The next time I will add a mix of 50:50 gum solution and water for better paintability. The bright side: I got considerably more than a pint after adding the water, a big difference from some other commercial glazes which are mostly water.

Friday 16th November 2018