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.
Context: G2934 - Matte Glaze Base for Cone 6, G2916F - Cone 6 Stoneware/Whiteware Glossy Base Glaze, A good matte glaze. A bad matte glaze., A functional matte cone 6 glaze should melt as well as a glossy, Cutlery Marking, Glaze Recipes
Tuesday 22nd May 2018
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
(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.
Context: Ferro Frit 3249, G2934Y - Cone 6 Magnesia Matte Low LOI Version, G2934 - Matte Glaze Base for Cone 6, G2934 vs. G2934Y cone 6 matte glaze, Matte Glaze, Dolomite Matte
Tuesday 22nd May 2018
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 craze
s, 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 craze
s, 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.
Context: IWCT 300F:Ice Water Crazing Test, Calculated Thermal Expansion, Crazing
Tuesday 22nd May 2018
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.
Context: GR6-A - Ravenscrag Cone 6 Clear Glossy Base, GR10-A - Pure Ravenscrag Slip, Ravenscrag Slip, Sterile white vs. pure Ravenscrag Slip as a liner glaze at cone 10R, Calcine, Calcination
Sunday 20th May 2018
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.
Context: Laguna B-Mix Cone 10R mugs with Alberta and Ravenscrag glazes, Laguna B-Mix with Ravenscrag GR10-A, GR10-C glazes and 10% add FireRed clay, Reduction Speckle, Reduction Firing
Saturday 19th May 2018
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.
Context: Opacity, Opacifier, Opacification
Friday 18th May 2018
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.
Context: G2934Y - Cone 6 Magnesia Matte Low LOI Version, Where Do I Start?, Brushing Glazes, Glaze Mixing
Tuesday 15th May 2018
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.
Context: Where Do I Start?, Glaze Mixing
Sunday 13th May 2018
Mel Noble at Plainsman Clay's Ravenscrag, Saskatchewan quarry
Plainsman extracts 6 different sedimentary clays from this quarry (Mel knows where the layers separate). The dried test bars on the right show them (top to bottom). The range of properties exhibited is astounding. The top-most layer is the most plastic and has the most iron concretion particles (used in our most speckled reduction bodies). The bottom one is the least plastic and most silty (the base for Ravenscrag Slip). The middle two are complete buff stonewares made by mother nature (e.g. M340 and H550). A2, the second one down, is a ball clay (similar to commercial products like OM#4, Bell). A2 is refractory
and the base for Plainsman Fireclay. The second from the bottom fires the whitest and is the most refractory
(it is the base for H441G).
Context: Plainsman Clays, Secondary Clay
Saturday 12th May 2018
Ravencrag rutile blue vs. Alberta Slip floating blue at cone 6
Both have been applied at moderate thickness on Plainsman M325 (using a slurry of about 1.43-1.45 specific gravity
, higher values end up getting them on too thick). The Ravenscrag version highlights contours better (the edges are black because of the black engobe
underneath). It also produces the blue color whether or not the kiln is slow cooled to 1400F (although a faster cool is less blue). But the Alberta Slip version has zero cobalt so is less expensive to make. It produces a deeper color over the black engobe
underneath the upper section of the pieces. Both of these produce a wide range of effects with different thickness, bodies and firing schedules.
Context: GA6-C - Alberta Slip Rutile Blue Cone 6, GR6-M - Ravenscrag Cone 6 Floating Blue, Plainsman Cone 6 Slow Cool (Reactive glazes)
Sunday 6th May 2018