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.
Context: Transparent Glazes
Sunday 2nd December 2018
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.
Context: CMC Gum, Alberta Slip, Dipping Glazes, Glaze Crawling
Thursday 29th November 2018
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.
Context: CMC Gum, Bentonite, Brushing Glazes
Friday 23rd November 2018
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.
Context: Epsom Salts, Suspending pure feldspar and applying it as a glaze, Pure Custer Feldspar and Nepheline Syenite on cone 10R porcelain bodies, Thixotropy, Powdering, Cracking and Settling Glazes
Friday 23rd November 2018
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.
Context: CMC Gum, Where Do I Start?, Brush-on commercial pottery glazes are perfect? Not quite!, Glaze Layering, Brushing Glazes
Friday 23rd November 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 brushing or base-layering versions using CMC gum.
Context: G2934Y - Cone 6 Magnesia Matte Low LOI Version, Where Do I Start?, Brushing Glazes, Glaze Mixing, Dipping Glazes
Friday 23rd November 2018
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.
Context: Firing Shrinkage, Drying Shrinkage
Tuesday 20th November 2018
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.
Context: Brushing Glazes, Specific gravity
Friday 16th November 2018
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