Black engobed M340 stoneware with GA6-B Alberta Slip glaze
Hand built. Cone 6 drop-and-holdPLC6DS firing. The engobe is the L3954B base recipe with added Mason 6600 black stain, it was applied at the leather hard stage inside and part way down the outside. The GA6-B glaze enhances the black under it. By Tony Hansen.
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.
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.
These are porcelains are made using Grolleg kaolin. P700 has 19% G200 Feldspar and Coleman (popular among potters in the US) has 31% (P700 switches the 12% to kaolin). Although the Coleman porcelain is more vitreous (top right) it is not more translucent. But it is a lot more problematic in fired warping and plucking (lower left). Grolleg kaolin has a high natural flux content so less feldspar is needed than in recipes calling for American kaolins. And that extra kaolin in P700 gives us something: More plasticity. Another thing: These are not as translucent as what can be achieved at middle temperature oxidation.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Measuring clay test bars done by Luke Lindoe 40 years ago
Luke Lindoe prospected Montana and Idaho for clays during the 1970s. He found an amazing variety of fireclays, earthenwares and stonewares. Every color, texture, plasticity. For each he made test bars to fire at different temperatures. Our M2 and Troy clays originated from this work. We just found these bars, but do not have Luke's shrinkage and porosity data, so are measuring them now. He code-numbered each and stamped them with four-inch marks. So we can derive the total fired shrinkages and measure the porosities. We can tell a lot about the plasticity of each by the nature of the cut lines. The texture also is obvious. Now we just need to start searching Luke's map archives to find out where all of these are.
Wednesday 3rd October 2018
We have to fight with the fibreglass industry to get kaolin!
These are bags from three recent truckloads of 880 bags each. Order-delivery delays are getting longer and longer as the fibreglass industry is making more and more demands on kaolin suppliers. This means we have to store this material in larger quantities and for longer periods than in the past. And we must be more diligent in testing for consistency because manufacturers are catering to fibreglass instead of ceramics. When this is coupled with the decline of ceramic manufacturing in North America it means maintaining and documenting the properties important to ceramics are becoming less important to kaolin manufacturers.
Here is what it takes to make sure P700 has minimal fired specks
We had to sample every pallet of a 1500 bag bentonite shipment. On testing each one we found dark-coloured particulates. Then we determined which pallets where the worst and did a second round of testing. Then we mixed up 5000 gram P700 test batches from three pallets, made ware and tiles, clear glazed and fired it all to cone 10R (with heavy reduction). We also prepared samples and returned them to the manufacturer for further testing in their lab. As it turned out, the dark particles were not iron-containing and we found only a few tiny specks.
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 L3954J 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).
This is G2934 matte (cone 6) and G2931K glossy base recipe having the same Cerdec yellow encapsulated stain been added (about 10%). You really need to see these side-by-side to appreciate how much richer the silky matte version looks (and feels). AMACO has popularized the bright glossy transparent look in its 'Celadon' product line (while the effect looks good on smaller areas, for cover glazing it appears 'washed out'). An additional factor to consider is that encapsulated stains begin to decompose around cone 6, this can produce orange-peel and blisters on glossy glazes, but with this matte base we have not seen it happen with any color. Google 'G2934 recipe' to learn more, it is being talked about everywhere, a truly amazing base.
Friday 21st September 2018
A mug cracks before your eyes because of glaze compression
A highly vitreous, thin walled mug is glazed inside-only. The glaze has a thermal expansion that is too low and it is under compression, pressing outward. A tap with a spoon is enough to trigger a sudden crack. It opens under the pressure. Had it been glazed on the outside also it would likely survive, but this test still indicates that it would be better to raise the expansion a little.