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What happens if Ravenscrag Raspberry is applied too thick

What happens if Ravenscrag Raspberry is applied too thick

These are Plainsman P300 mugs fired at cone 6. When the glaze, GR6-E, goes on too thick (as on the left) it is dark maroon and has a pebbly surface that does highlight contours. This went on too thick because the specific gravity of the slurry was too high, about 1.53 (even a one-second dip put to thick a layer on the pieces). When I thinned it down to about 1.45 and flocculated it using espom salts, it went on thinner, yet still evenly, and I got the result on the right.

Saturday 16th June 2018

Coffee Clay now available

Coffee Clay now available

Left: M390. Right: Coffee. The glaze is GA6-A Alberta Slip amber base. The top halves have L3954B white engobe under the glaze. To read all about it on the Coffee Clay data sheet click here. Its working properties are like M340. Glazes that work on M340 should work on this (e.g. G2926B clear, Alberta Slip and Ravenscrag Slip glazes). Colorants in the body will bleed into glazes, making them appear completely different than they would on lighter burning bodies. Rutile glazes will come alive! Most transparent glazes will bubble-cloud, use GA6-A if possible (see the data sheet for more information).

Thursday 24th May 2018

Tune your matte glaze to the degree of matteness you want

Tune your matte glaze to the degree of matteness you want

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-crystals, 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 GBMF test 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 added (100% far right). Bottom: G2916F in the same proportions. The effects are similar but the top one produces a more pebbly surface.

Tuesday 22nd May 2018

Matte cone 6 glazes have identical chemistry but one melts more. Why?

Matte cone 6 glazes have identical chemistry but one melts more. Why?

These are 10 gram GBMF test balls that we melted on porcelain tiles at cone 4 (top two) and cone 6 (bottom two). They compare the melt fluidity of G2934 (left) and G2934Y (right). The Y version sources its MgO from frit and talc (rather than dolomite). It is a much more fluid melt because the frit is yielding the oxides more readily. But Y has a key benefit: It has a much lower LOI, producing fewer entrained air bubbles and therefore fewer surface defects. And, even though it runs much more, it has the same matte surface! As long as it is applied at normal thickness, the extra melt fluidity does not cause any running. And it has another benefit: Less cutlery marking issues. It is actually a very durable and practical food surface glaze, having a low thermal expansion that fits almost any body. Although these appear glossy here, on ware they have the identical pleasant silky matte surface.

Tuesday 22nd May 2018

Match calculated COE to dilatometer-measured body COE? No!

Match calculated COE to dilatometer-measured body COE? No!

Why? Firing temperature, schedule and atmosphere affect the result. Dilatometers are only useful when manufacturers monitor bodies AND glazes over time and in the same firing conditions. Calculated values for glazes are only relative (not absolute). The best way to fit glazes to your clay bodies is by testing, evaluation, adjustment and retesting. For example, if a glaze crazes, adjust its recipe to bring the expansion down (your account at Insight-live has the tools and guides to do this). Then fire a glazed piece and thermal stress it (300F-to-ice-water IWCT test). If it still crazes, move it further. If you have a base glossy glaze that fits (and made of the same materials), try comparing its calculated expansion as a guide. Can you calculate body expansion from oxide chemistry? Definitely not, because bodies do not melt.

Tuesday 22nd May 2018

Roasting Ravenscrag Slip instead of calcining

Roasting Ravenscrag Slip instead of calcining

This is the Ravenscrag Slip I used to calcine at it 1850F (about 10lbs in a bisque vessel). I am now roasting it to 1000F instead, this produces a smoother powder, less gritty. I hold it for 2 hours at 1000F to make sure the heat penetrates. It is not actually calcining, since not all crystal water is expelled, so we call it "roasting". Why do this? Ravenscrag Slip is a clay, it shrinks. If the percentage is high enough the glaze can crack on drying (especially when applied thickly). The roast does not shrink. The idea is to tune a mix of raw and roast Ravenscrag to achieve a compromise between dry hardness and low shrinkage. Technically, Ravenscrag losses 3% of its weight on roasting so I should use 3% less. But I often swap them gram-for-gram.

Sunday 20th May 2018

Laguna B-Mix on Steroids! I have wedged in 10% and 20% Plainsman FireRed

Laguna B-Mix on Steroids! I have wedged in 10% and 20% Plainsman FireRed

Both pieces have a transparent glaze, G1947U. FireRed (the bar in the front) is a mix of A1 and St. Rose Red, both having heavy ironstone concretions. BMix has some specks anyway, this adds thousands creating some awesome aesthetics. This addition does not affect the working properties of BMix, it still throws very well. An added benefit is that pieces dry better. Fired strength and maturity are minimally affected (porosity stays around 1%). With a 20% addition the surface of the unglazed clay is almost metallic. Silky matte glazes, like G2571A, are stunning on a body like this.

Saturday 19th May 2018

The right amount of opacity highlights the incised design

The right amount of opacity highlights the incised design

The mug on the left is a commercial brushing glaze. The mechanism of this effect is that the glaze is much thinner on the edges of the design, thin enough that its opacity is mostly lost. The potter is attempting to mix her own equivalent (center and right). Her glaze adds 4% tin oxide to a transparent. However, as you can see, she has added too much. Further testing using lower percentages will find the right balance between the opacity needed to cover the brown body on the flat areas and the transparency needed to expose it on the contours.

Friday 18th May 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 a brushing version using CMC gum.

Tuesday 15th May 2018

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

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 your cover and liner glazes 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.

Sunday 13th May 2018

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