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
GA6A Alberta Slip base using Frit 3124, 3249 and 3195 on dark body
The body is dark brown burning Plainsman M390 (cone 6). The amber colored glaze is 80% Alberta Slip (raw:calcine mix) with 20% of each frit
. The white engobe
on the inside of two of the mugs is L3954A (those mugs are glazed inside using transparent G2926B
). The Alberta Slip amber gloss glaze produces an ultra-gloss surface of high quality on mugs 2 and 3 (Frit
3249 and 3195). On the outside we see it this glaze on the white slip until midway down, then on the bare red clay. The amber glaze on the first mug (with Frit 3124) has a pebbly surface that is not working nearly as well. These mugs are fired using a drop-and-soak
firing schedule. Some caution is required with the 3249 version, it has low thermal expansion
(that is good on bodies that normally craze
glazes, but risks shivering
on ones that do not).
Context: Ferro Frit 3249, GA6-A - Alberta Slip Cone 6 Amber Base Glaze, GA6A Alberta Slip base using Frit 3249 and 3195 on buff body, P300 and M370 mugs with GA6A Alberta Slip (using Frit 3249), Alberta Slip Cone 6 Base with Frit 3249, 3195, Plainsman Cone 6 Slow Cool (Reactive glazes)
Saturday 28th April 2018
Roasting Alberta Slip at 1000F
Roasted Alberta Slip (right) and raw powder (left). These are thin-walled 5 inch cast bowls, each holds about one kg. I hold the kiln at 1000F for 30 minutes. Why do this? Because Alberta Slip is a clay, it shrinks on drying. Roasting eliminates that, a 50:50 raw:roast mix works well for most recipes having high percentages of Alberta Slip. And 1000F? Calcining to 1850F sinter
s some particles together (creating a gritty material) while 1000F produces a smooth, fluffy powder. Technically, Alberta Slip losses 3% of its weight on roasting so I should use 3% less than a recipe calls for. But I often just swap them gram-for-gram.
Context: Alberta Slip 1900F Calcined, Alberta Slip 1000F Roasted, GA10x-A - Alberta Slip Base for cone 10 oxidation, GA6-A - Alberta Slip Cone 6 Amber Base Glaze, GA6-C - Alberta Slip Rutile Blue Cone 6, GA10-B - Alberta Slip Tenmoku Cone 10R, GA10-D - Alberta Slip Black Cone 10R, GR10-E - Alberta Slip:Ravenscrag Cone 10R Celadon, GA6-D - Alberta Slip Glossy Brown Cone 6, L3341B - Alberta Slip Iron Crystal Cone 10R, GA6-G - Alberta Slip Lithium Brown Cone 6, GA10-A - Alberta Slip Base Cone 10R, GA6-G1 - Alberta Slip Lithium Brown Cone 6 Low Expansion, GA6-AR - Alberta Slip Cone 5 Reduction Base Glaze, GA6-F - Alberta Slip Cone 6 Oatmeal, GA6-H - Alberta Slip Cone 6 Black, Calcine, Calcination
Saturday 28th April 2018
Twenty six bodies. Porcelains and native. Which do I like best?
I am testing runs of clays we (Plainsman Clays) make for potters. We are doing too many small-run products, bodies that we want to discontinue because others we are really good at making are much better. I am using native bodies and porcelains but the native ones will turn out best. I got lots of s-cracks on the porcelains, the low humidity at this time of the year caught me by surprise. But I got zero cracks on native bodies. And I am using engobe
s on the natives, these can turn even a dark colored stoneware surface pure white (I am also using a black engobe
here). I'll use a mix of base and cover glazes that I make myself (using recipes we publish) for food surfaces and decorate some using bottled commercial glazes.
Monday 16th April 2018
Absolutely jet-black cone 6 engobe on M340
This is the L3954B engobe
recipe but it has 15% Mason 6600 black body stain (instead of the normal 10% Zircopax for white). There is no cover glaze, yet it is durable and absolutely coal black (so a lesser stain % is possible). We have updated the mixing instructions at PlainsmanClays.com and Digitalfire.com pages (showing exact amounts for water, powder, Darvan) and the text on the glossary pages about thixotropy
s (read these again and learn to use the engobe process even better). This engobe base is designed to work on regular M340/M390 stonewares (not porcelains). This is exciting because these bodies are so much more robust in drying and much less expensive than porcelains.
Context: L3954B - White Cone 6 Engobe for Plainsman M390, M340, L3954B engobe page at PlainsmanClays.com, Engobe, Thixotropy
Friday 13th April 2018