Things I've learned: Glaze edition
Jul. 15th, 2017 09:49 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
offcntrI've been poking around the archives, trying to find the posts on glaze and overglaze recipes, because every now and then someone asks for them. I've finally found both, and have retagged them glaze recipes, so they'll be easier to find.
But recipes aren't the only important information. There are details of mixing and application that need to be considered.
Consistency. Glaze recipes tend toward dairy metaphors. A glaze should be the consistency of milk, cream, half-and-half. I found this maddeningly imprecise, and a real problem when painting on vertical surfaces. Too thin a glaze coat, and the base is brown and unattractive from the iron in the clay body, but too thick, and the picture runs off the pot. After far more years than I'd like to admit, I finally bought a hydrometer. It's a glass tube, like an oversized thermometer, with a lead weight in the bottom and a paper scale, graduated to measure specific gravity. After mixing the glaze thoroughly and waiting for it to stop spinning around in the bucket, I lower the hydrometer into the glaze and let it float, then read off the scale where it's at the surface level. The specific gravity of water is 1.000; I've found 1.500 works perfectly for me. (As an aside, this means there's twice as much water as dry materials in my glaze, so for my standard batch of 14,000 grams, I'm actually mixing up 42,000 grams of glaze--a bit over 90 lbs.)
I also find that the glaze gets thicker with use, whether from evaporation or absorption of water into the bisque, so I check a couple of times a day, and add water as necessary.
Thickness. The thickness of application of a glaze depends on several factors: how you apply it, for how long, and how much water the pot can absorb. I dip or pour the glaze--spraying leaves too fluffy a surface for my overglaze decoration--for a count of four (one, two, three, four--out again), which I long ago determined is actually about three seconds. Apparently, I count in waltz time, 3:4. Smaller pots, especially thinner pots like soup or toddler bowls, may need a little more time in the glaze to get a proper coating. I usually dip lids for casseroles, cookie jars, honey pots twice to get a proper coating.
Settling. If the glaze isn't fully suspended, you're not going to get an even coat. If your glaze is high in slightly soluble minerals, like nepheline syenite, it'll want to settle to the bottom of the bucket and form a rock there, while leaving just enough in suspension to fool you into thinking you're still applying glaze. Mix early, mix often. And a teaspoon of epsom salts, and a bit of aging, doesn't hurt either. I now mix my glaze for each firing during the previous firing--four to six weeks before I actually start glazing again. The extra conditioning time does wonders for glaze suspension.
Overglaze application. If you're used to underglazes, you're gonna put the stains on too thickly. You want it the consistency of india ink, maybe a little thinner. And that's for the initial line drawing, at full strength. Dilute with water to create washes, shadows, lighter tones. There's almost homeopathically small quantities of red stain in the water when I glaze bunny ears. I want only the palest of pink. Some colors have more tinting strength than other. Cobalt carbonate can be diluted many times and still give you blue.
If your stains are blistering, or, worse, burning out to a scabby grey scale, you'll really want to dilute them down some.
Firing temperature. There's a lot of variation in the way potters fire to cone 10. I know a potter who fires cone 10 flat on the pad, 11 nearly touching down to get the glaze effects he wants. If I did that, every picture on every vertical surface would be a blurred smear. I want cone 10 just touching at its tip. If the kiln is firing unevenly, I may fiddle with the damper to try and catch up the cold end, but not past that point. I can usually count on carry-over to drop that last half-cone, particularly if the top of the kiln is cooler than the bottom at shut-down.
Other firing ranges. I'm firing cone 10 in reduction; I've also used cobalt, iron and rutile at cone 6, reduction. Not sure how they'd work in oxidation; some metals, iron in particular, act as a flux in reduced form, but a refractory in oxidation. Experiment with a reliable white/whitish base glaze, moderately glossy, and be prepared to add Gerstley Borate or Ferro Frit 3134 to your oxides and stains. I wouldn't overglaze on a matte glaze if I intended to use it for food; I'd worry that the oxides wouldn't be sufficiently absorbed into the glassy matrix.
But recipes aren't the only important information. There are details of mixing and application that need to be considered.
Consistency. Glaze recipes tend toward dairy metaphors. A glaze should be the consistency of milk, cream, half-and-half. I found this maddeningly imprecise, and a real problem when painting on vertical surfaces. Too thin a glaze coat, and the base is brown and unattractive from the iron in the clay body, but too thick, and the picture runs off the pot. After far more years than I'd like to admit, I finally bought a hydrometer. It's a glass tube, like an oversized thermometer, with a lead weight in the bottom and a paper scale, graduated to measure specific gravity. After mixing the glaze thoroughly and waiting for it to stop spinning around in the bucket, I lower the hydrometer into the glaze and let it float, then read off the scale where it's at the surface level. The specific gravity of water is 1.000; I've found 1.500 works perfectly for me. (As an aside, this means there's twice as much water as dry materials in my glaze, so for my standard batch of 14,000 grams, I'm actually mixing up 42,000 grams of glaze--a bit over 90 lbs.)
I also find that the glaze gets thicker with use, whether from evaporation or absorption of water into the bisque, so I check a couple of times a day, and add water as necessary.
Thickness. The thickness of application of a glaze depends on several factors: how you apply it, for how long, and how much water the pot can absorb. I dip or pour the glaze--spraying leaves too fluffy a surface for my overglaze decoration--for a count of four (one, two, three, four--out again), which I long ago determined is actually about three seconds. Apparently, I count in waltz time, 3:4. Smaller pots, especially thinner pots like soup or toddler bowls, may need a little more time in the glaze to get a proper coating. I usually dip lids for casseroles, cookie jars, honey pots twice to get a proper coating.
Settling. If the glaze isn't fully suspended, you're not going to get an even coat. If your glaze is high in slightly soluble minerals, like nepheline syenite, it'll want to settle to the bottom of the bucket and form a rock there, while leaving just enough in suspension to fool you into thinking you're still applying glaze. Mix early, mix often. And a teaspoon of epsom salts, and a bit of aging, doesn't hurt either. I now mix my glaze for each firing during the previous firing--four to six weeks before I actually start glazing again. The extra conditioning time does wonders for glaze suspension.
Overglaze application. If you're used to underglazes, you're gonna put the stains on too thickly. You want it the consistency of india ink, maybe a little thinner. And that's for the initial line drawing, at full strength. Dilute with water to create washes, shadows, lighter tones. There's almost homeopathically small quantities of red stain in the water when I glaze bunny ears. I want only the palest of pink. Some colors have more tinting strength than other. Cobalt carbonate can be diluted many times and still give you blue.
If your stains are blistering, or, worse, burning out to a scabby grey scale, you'll really want to dilute them down some.
Firing temperature. There's a lot of variation in the way potters fire to cone 10. I know a potter who fires cone 10 flat on the pad, 11 nearly touching down to get the glaze effects he wants. If I did that, every picture on every vertical surface would be a blurred smear. I want cone 10 just touching at its tip. If the kiln is firing unevenly, I may fiddle with the damper to try and catch up the cold end, but not past that point. I can usually count on carry-over to drop that last half-cone, particularly if the top of the kiln is cooler than the bottom at shut-down.
Other firing ranges. I'm firing cone 10 in reduction; I've also used cobalt, iron and rutile at cone 6, reduction. Not sure how they'd work in oxidation; some metals, iron in particular, act as a flux in reduced form, but a refractory in oxidation. Experiment with a reliable white/whitish base glaze, moderately glossy, and be prepared to add Gerstley Borate or Ferro Frit 3134 to your oxides and stains. I wouldn't overglaze on a matte glaze if I intended to use it for food; I'd worry that the oxides wouldn't be sufficiently absorbed into the glassy matrix.