Glazing Pottery

28 Sep

I’m constantly asked by students (most of them under the age of 12), “Are we going to paint the pottery today?” Cute, and I always hate to disappoint them, but many adults don’t know a lot more about finishing pottery than the kids do.

Pottery can certainly be painted, and many people do that–I do sometimes myself. Here’s an example of a little dragon flute that I sculpted and my daughter painted.

We decided to paint it because Cheri likes the metallic colors she used for little Draquina, and I think she turned out really well. She only plays a quatrain, which is why I have officially coined the term quaturina in place of ocarina.

Anyway, yes, you can paint pottery and no, it’s not some sort of heresy to do so. Typically, you would paint pottery that has been fired to the bisque stage, and if you want to do this, you’ll also want to prime the pottery first with any type of inexpensive acrylic paint.

Most of the time, you’ll be glazing, not painting your pottery, though you might apply the glaze or underglaze with a paint brush. Most potters wait to glaze their pottery until it has been bisque fired, though it can be glazed when leather hard or bone dry.

Mid and high fire glazes are not usually applied to the underside of pottery. They melt and stick the pot to the shelf. If you’re glazing a lightweight piece, you may be able to use kiln stilts to keep a fully glazed piece from sticking, but in general, anything but low-fired pottery is left unglazed where it touches the shelf.

Let’s start by talking about underglaze, just to make things simpler. Underglaze, as you would suppose, is applied under a glaze, usually a clear glaze. You paint on the underglaze with a brush, a sponge, a splattering tool, or whatever you like, and then dip or spray the pot with a layer of clear glaze. Underglazes can resist the clear glaze if you paint them on too thickly, and you’ll end up with a pot that has a matte surface that isn’t very durable.

You can also buy or make glazes that are designed to be painted on with a brush. Glazes are different than underglazes in that they are a complete finish in themselves. Underglazes are not complete in themselves, but must be covered with a glaze. So, the brush-on glazes . . . they come in little jars and you just put them on with a brush. You get a lot more control this way than you would if you were spraying or dipping your pots, but it’s also a lot slower work. Most brushable glazes are designed to be applied in 2-3 coats. They should end up about the thickness of a credit card, so if your glazes are going on thicker than this, you need to thin them out by mixing in a little water.

The most popular way to glaze functional pieces such as cups and bowls, etc., is to dip the pottery pieces in a bucket of glaze slurry. This type of glaze, as well as many of the aforementioned brushable glazes, is not usually anything like the color of the finished product. A gray or dark brown glaze might fire up blue. Green glazes are often colored with copper oxide and are therefore usually green in the bucket, but that won’t tell you anything about the shade of green. White glaze is also white in its raw state, but so is clear, unless it’s been colored blue to identify it and differentiate it from the white. The blue will burn out and will not affect the color of the finished glaze.

Dipping glaze comes in liquid or powdered form. Liquid is obviously easier, but more expensive to ship. It may or may not arrive properly thickened or thinned, but we’ll get into that later. I make my own glazes in 10 kg batches, but if I were buying, I’d either buy pre-mix or purchase 25lbs of glaze (which is a little more than 10 kg). This amount will make 3.5-4.5 gallons of glaze, depending on the formula of the glaze. To mix up your 25 lbs of powdered glaze, you’ll need the following:

  1. Five gallon bucket
  2. Stirring stick
  3. Sieve and stiff brush
  4. Dust mask
  5. Power drill and 5 gallon paint stirring attachment (optional)

It’s best to work outside if at all possible, as the dust from powdered glaze can float in the air for quite a long time. Otherwise, be sure to work in an area with an adequate exhaust fan–adequate means really, really powerful, by the way. If you can’t secure a suitable place to mix glaze, it’s best to buy it pre-mixed. Here’s the procedure.

  1. Put about two gallons of water into the bucket. You’ll probably need more, but it’s easier to add it than to take it away.
  2. Pour in the glaze powder in a slow, steady stream if possible. This is hard to do, by the way, and most likely the glaze will all flumph out at once and splash you, but you’ve gotta try.
  3. Mix the glaze with the stick first, then, if you have it, with the power drill. If it’s awfully thick, add some water. Let it sit for an hour or two if you can, then mix it again and pour it through a sieve. You’ll need to work the glaze through with a stiff brush. Mix well and re-sieve, several times until the glaze has reached a creamy and smooth consistency.
  4. Next day, re-mix the glaze and assess the thickness. For a baseline, you can dip your finger in and out in an unhurried, but business-like motion. If you can still see the lines of your knuckle and fingernail clearly, but the finger is completely coated, your glaze is within the range of proper thickness. If the glaze is too thin, you’ll have to let it sit until it separates, then you can dip water off the top. If it’s too thick, of course, you just add water.

If a glaze has been sitting for several months, it’s usually a good idea to sieve it before using, and all dipping glazes need sieving periodically. To glaze a pot by dipping, you can either keep glaze from sticking to the base by applying wax or just by not dipping the base. Alternatively, you can wipe the glaze off the base on a piece of damp carpet. I use a mix of all three techniques, depending on the piece to be glazed. Here’s the procedure:

  1. Any parts of the pot where I don’t want glaze–decorations, lid flanges, etc., I apply wax to. It takes a while for the wax to dry, so be sure to do this several hours in advance of glazing.
  2. If the base of the pot is going to be dipped in the glaze, I first dip it into water to keep too much glaze from sticking there.
  3. Have a piece of damp carpet available. I put it in a large disposable baking pan to keep it from messing up the table. Using a pair of glaze tongs (available at a ceramic supply outlet), dip your pot sideways into the glaze and remove it promptly, emptying it out of glaze. Place the piece base down onto the carpet, and when the glaze has dried sufficiently, hold the piece by the rim and twist it back and forth until the base of the piece is clean.
  4. Alternatively, if it’s possible, you can hold the piece by the base and dip it straight down into the glaze, stopping before you immerse the base. If it’s a cup or bowl that needs the inside glazed, too, you can squirt glaze inside with a turkey baster or pour in with a cup of some sort, swirl to coat, and then pour out.
  5. It’s important to keep the glaze as thin as possible near the base of the pot, so it’s not so likely to run and drip during firing, sticking the pot to the kiln shelf.
  6. To vary your glaze’s appearance, you can dip the rim of an already glazed pot with a second color. You can drip a second glaze on with a turkey baster or splatter it on with a toothbrush. Be sure you take careful notes so you’ll remember what you did. That way you can repeat a stunning combination or avoid repeating a dud.

Glazes designed for dipping can be painted on, but they don’t flow well, which makes them difficult to work with in this way. They can also be sprayed. I’m not going to go into this here, though. Spraying requires quite a lot of special equipment and so isn’t going to be accessible to very many beginners. I may cover it in a later post.

As with painted glazes, you want the coating to be about the thickness of a credit card, though different glazes require more or less coverage. If your glaze doesn’t show the color, or much of the color you expected, you’ve probably applied it too thinly. Take some water off the glaze, immerse your pot for a longer period of time, or double dip it. If the glaze drips, next time be careful about getting too much glaze close to the base or getting the glaze/glazes on too thick.

Glaze that develops a network of cracks as it dries is too thick. You can heal the cracks by gently rubbing over them with a damp sponge or a damp finger, but if the thickness is heavy near the bottom of the pot, you need to remove some of the glaze there, whether by scraping or washing the area. If this situation is too bad, you may want to wash the pot, let it dry completely, and re-glaze another time.

If the glaze on your pot is too thin, it’s possible your glaze slurry wasn’t well mixed. Glaze slurries need frequent mixing as they settle. Some settle toward the bottom, and others have particles that tend to glob together toward the top of the bucket. Either way, they require frequent stirring. A pot that has been glazed too thinly because of a badly mixed glaze will have soaked up quite a bit of water and will need to sit for a while before you try again. When you do re-dip the pot, just dip the top 3/4 or so, to avoid too thick a glaze layer close to the base.

There’s a lot more than this to know about glazing, but this will get you started. Many books, thick books, have been written on this subject. Just dive in and don’t be afraid to experiment. I’ve tried to point out a few of the speed bumps, but you’ll have both spectacular successes and miserable failures. Don’t give up and you’ll soon have a good understanding of your own particular favorite glazes.

2 Responses to “Glazing Pottery”

  1. David Essex August 24, 2010 at 10:16 am #

    Im probally being really silly but if you get the glaze wrong and it starts to crack, do you just reglaze it without doing anything else or do you try to remove the glaze before hand or maybe theres something else your meant to do ?

    • cindyinsd August 25, 2010 at 12:20 am #

      Not silly at all, David. The simplest answer is that your glaze is on too thick. That’s not always the case, however. If the glaze is too thick, but not by a lot, you can rub over the cracks with a damp finger and tie them back together before you fire. (If you don’t, the glaze is likely to crawl — that is, it will puddle away from the cracks.) If the glaze is on the outside of the pot, you risk having it run onto the shelf; if it’s on the inside, you risk having a broken surface at the bottom that looks like it boiled and then froze, which is likely what it did. However, it may be okay — if it’s only a wee bit too thick.

      If the cracked glaze flakes off as you try to rub it, you can either wash it off and start over, or you can take a chance and see what it ends up looking like. Of course, taking a chance is interesting and fun, but protect your shelves just in case. You can put a bisqued disk underneath your pot or just slop on some extra kiln wash. I would also stilt the pot in some way. That way you can grind off the drips with an angle grinder (masonry disk) instead of the pot breaking because it stuck to the shelf. (If you wash it off, let it dry completely before reglazing!)

      Some glazes are badly formulated and seem to crack no matter what you do. You can either accept this and use the crawling as a decorative feature or throw the glaze away. It’s difficult to adjust if you don’t know what’s in the glaze. Try adding a bit of water to thin the glaze before you toss it, though, and see if that helps.

      Some glazes are designed to crawl, and they’re supposed to crack as they dry. This isn’t a good feature for functional pottery, but can be a plus for sculptural pieces that need that kind of surface. Do a web search for crawling glaze images and I’ll bet you find some examples that will give you an idea what a crawling glaze looks like.

      Hope this helps you!

      Blessings, Cindy

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