Glaze Firing Pottery

14 Oct

You’ve got your pottery all covered in promise–that is to say, glaze. The glaze will either make or break your piece, so be sure to follow my suggestions in the post on glazing as well as reading and following any instructions that might have come with the glaze, stack your kiln carefully (nothing touching anything else, no glaze touching shelves or walls), pray, and stay within reasonable proximity to the kiln.

We’re talking about electric firing here, btw. Choose a day in which you don’t expect the power to go out. That might sound obvious, but starting your glaze kiln on a dark and stormy day could have unfortunate complications. First, the barometric pressure does make a difference in the outcome of your firing. Second, if you’re using a computer controller, lightning is not your friend. You’ll be biting your nails until you’re able to turn both the kiln and its breaker off (or pull the disconnect switch). Third, even if the power just peacefully dies without a surge, it’s better not to interrupt a firing. Doing this can have permanent consequences to your wares–sometimes good, more often bad.

Read my precautions about kiln venting given in my post on bisque firing. In fact, read the whole post again. There’s a lot of stuff there that applies equally to glaze firing. Directions for firing differ from kilns controlled by a sitter to kilns controlled by a computer device and its requisite pyrometer. (If you don’t know what I’m talking about, read the bisque firing post.) I’m going to put the instructions for the sitter in black and those for the computer controller in italic.

One caveat: make sure your glazed ware is dry. If you think your ware may have residual moisture left over from glazing, start your firing on low power for an hour or so.

  • I often start a glaze kiln on high power and leave it there for the whole firing, but the sitter kiln I use is at the art & science center where I teach, and I’ve made it a point not to have any sensitive glazes there. If you have glazes that need a soak at the peak temperature, you might want to turn down one or two of the elements when the firing is nearly done. You’ll know this from your past records and from the behavior of the cone packs you have placed in the kiln.
  • Some people do recommend a slower start, even for a glaze kiln, but I typically start my kiln out at full power–that is, I tell it to heat at 500 degrees Fahrenheit per hour. It will be able to do this at first, but soon it will have to slow down because it can’t deliver enough power to rise that fast. The speed at which your kiln is able to heat depends on the elements, power available to them, size of the kiln, ambient temperature, and probably the phase of the moon.
  • If you’d like to soak your sitter kiln, wait until the target cone (in my case, 5 or 6) just begins to bend, then turn one or more elements down to medium. Leave them like this for the duration of your desired soak (usually about 30 minutes or less), then, if necessary, turn them up again. Ideally, you shouldn’t have to turn them up. You should learn, by experimentation, to turn the elements down just enough so that you will slow the rise of the kiln, but not stop it.
  • If you don’t yet know what temperature will cause your target cone to bend, you’ll need to watch the shelf cones closely as the kiln approaches peak temperature. A thermocouple may read 2260 degrees F. and yet the cone 6 on your shelf may show that the firing has achieved cone 6 temperature/heat work. Set the controller to slow your firing down when it reaches about 50 degrees below your target temperature. I tell it to fire at 50 degrees an hour at this point. Then, if you like, you can also program a soak at peak temperature of 15-30 minutes. You don’t want to soak too long, or your glazes will be in danger of running off onto the kiln shelves, even if you haven’t overfired them. At these temperatures, they will have become a viscous liquid, like molasses or honey, and too much time allows them to flow into places you’d rather not have them go.
  • For all practical purposes, you are done firing if you’re using a sitter kiln. Leave the vents turned on and keep an eye on things until the kiln has cooled enough not to present a danger of fire, then just allow it to cool naturally. When it is no longer warm to the touch, you can check to see if it has cooled enough to unload. Ideally, you should wait until ware and kiln furniture can be held in the bare hands for any length of time without discomfort from heat. Once you’ve been doing pottery for a while, you can think about turning the kiln back on at around 1900 degrees and thus slowing down the cooling to encourage microcrystaline matte glazes, but that’s a very advanced technique without a computer to help you and requires knowledge you can only gain from experience.
  • If your glazes are all glossy ones, you, too, can just turn the kiln off and let it cool at this point. If you have microcrystalline matte glazes in the kiln, set it to fire down. It should be allowed to cool naturally (just program in a high number–say 500) until it reaches 1900 degrees. You should then have it cool at 100-200 degrees an hour until it reaches 1700 degrees. At this point, you can allow the kiln to cool naturally. Just don’t program another ramp, and it will end the firing. As above noted, it is best to allow the kiln to cool until ware can be easily and comfortably be handled with bare hands. You’ll have to learn by experimentation how fast to cool the kiln during the 1900-1700 phase, and it will likely differ from glaze to glaze. A longer cooling makes a more matte glaze, but you can cool so slowly that normally buttery mattes will become almost dry, and that may or may not be what you were after.

A quick word about unloading a hot kiln. I confess that I have unloaded kilns that were so hot they made my welding gloves smoke and I had to take them off and let them cool before I could continue. This is considered bad practice, but after being patient for about so long, it’s sometimes more than I can do to be patient for another six hours or so. So . . . if you choose to ignore my good advice (as I sometimes do) . . .

  • Be sure to set your hot pottery on a suitable surface–like the lid of another kiln or a fire brick or a fairly warm kiln shelf.
  • Remember that thermal shock (sudden change of temperature) is the mortal enemy of pottery and glassware. Do not allow your pottery to contact a cold surface. If you have a history of Raku, know that raku clay is especially formulated to withstand thermal shock, and it sometimes breaks anyway.
  • Do not remove pieces with large flat bottoms from the kiln–plates, platters, etc.–because they will break. They’ll crack right in two. I promise.
  • Do not set hot kiln furniture (or hot pottery) on a vulnerable or a cold surface.
  • Wear protective clothing. The best heat-proof gloves you can afford, long-sleeve, natural fiber shirt, and tie long hair back.
  • Finally, really–don’t do it. You can wait. The pottery isn’t going anywhere.

So, I’m afraid this last bit of advice is like handing a kid a condom and telling him not to have sex, but you’ll make your own decisions about this sort of thing. I did. So, you might as well learn from my mistakes so that you can go on and make new mistakes of your own. 😉

Your pottery is out. Some of it is beautiful; some of it disappointing; and some of it may even (God forbid) be stuck to the shelf. Most of it will need sanding on the bottom so that it won’t scratch your furniture. Just use regular sandpaper from the woodworking section of the hardware store. I like 80 grit, but any coarse grit will work. If you have a power sander of any kind, you can use that, but be careful and follow appropriate precautions given by the manufacturer of the tool.

If you have glaze drips on your pottery or on your kiln shelves and furniture, you can use an angle grinder with a masonry disk to clean them off. Again: thick leather gloves, goggles, long sleeves, tie back your hair, use ear plugs, dust mask. Really! Especially the goggles and the gloves and the hair tie.

Making pottery well requires a long and patient learning process (or at the very least, a helpful teacher/mentor.) I know all of this sounds complicated, but it’s no more so than any other skill and a lot less than many. It’s a lot easier than building a house, for example. Enjoy the process; don’t get too attached to any individual piece; remember it’s just mud and you can make another one. 🙂

God bless,

Cindy

10 Responses to “Glaze Firing Pottery”

  1. GURUKUMAR October 22, 2008 at 10:43 am #

    hello sir
    i am practising glazing but i am getting more and more rejections for ex the ware will be dull,evaporation,pin holes and spots etc,
    i want to control it to a much extent.give me hint to have control on it.
    thank u

  2. cindyinsd October 22, 2008 at 8:30 pm #

    Dear Gurukumar,

    Many things can cause glaze faults.

    Dull ware can be the result of an underfired glaze,
    a glaze that has been applied too thinly,
    or a glaze that is badly formulated.

    Glazes that are underfired never completely melt, therefore, they are matte and not shiny. Some glazes are matte because they are designed to be matte and their matteness is caused by micro crystals. Glazes that are supposed to be glossy but turn out matte may have been underfired.

    Glazes that are applied too thinly may be dull in color and/or in shine. Most glazes should be applied so that they are as thick as a credit card. Some glazes need a slightly thicker application, and some (especially clear and white glazes) may need to be applied more thinly.

    Many glazes are badly formulated. Unless you know the recipe for the glaze, there’s not much you can do to fix them. Even if you do know the recipe, fixing them may be too difficult to make it worth the trouble. You can find much information on formulating glazes on the internet. A good place to start is Digital Fire. In addition to offering products for purchase, you will find many free articles on Digital Fire’s website.

    I am not sure what you mean by “evaporation.”

    Pin holes are a common problem with many possible causes. First, your ware may be dusty. Even if it has just come from bisque firing, it is a good idea to wipe it with a damp (not wet) sponge before glazing it.

    Second, your ware may have gasses left inside that have not been exhausted during bisque firing. To eliminate this problem, you can hold your ware at the peak of your bisque firing for 30 minutes, or you can bisque fire it a little hotter. Be advised that if you bisque hotter, your ware will absorb less glaze during the glazing process, so be sure the glaze coat is thick enough.

    Third, your glaze may be very thick and viscous at your peak glaze firing temperature. Bubbles that form try to escape, but the surface of the glaze does not heal because it is too thick to flow back into the hole left by the bubble. One way to solve this is to “soak” your glaze kiln at the peak temperature for 30 minutes or longer. Another way to solve this is to reformulate the glaze so that it is not so thick at the peak temperature.

    As for spots, I am not certain what you mean by this. Usually spotting is caused by colorants in the clay itself. These colorants are added to manufactured clay for the purpose of causing spots in the finished ware. In native clay, the spotting agents may be unwanted contaminants. Usually they consist of larger particles of iron and/or manganese.

    For a thorough treatment of glaze flaws, I recommend Ceramic Faults and their Remedies by Harry Fraser.

    I hope this helps, Gurukumar. Best of luck with your work.

    Cindy

  3. Beth January 23, 2009 at 8:27 pm #

    Speaking of firing…can you tell me what to look for in a used kiln? I’m looking at a Skutt Model 181.

    Any general info would be great!

    Many thanks,
    Beth

    • cindyinsd January 23, 2009 at 10:12 pm #

      Hi, Beth

      Skutt is a good brand, from what I’ve heard, but I haven’t used a Skutt kiln. In buying a new kiln, you’d want to do a tremendous amount of shopping around and research, but with a used kiln, you don’t have nearly the range of choices. I would recommend calling Skutt and asking them about the kiln, what you might want to check out, etc. Many of the owners of these ceramic companies are very helpful, having started out as ceramists themselves.

      You’ll want to examine the condition of the fire brick (the inside walls of the kiln plus the floor and lid). There will probably be bits broken off if the kiln has been used much, but it shouldn’t be in horrible condition. There will likely be a crack running across the floor, but as long as the floor seems stable, this isn’t a big deal. The hinging mechanism for the lid should be in good shape. There isn’t much that can’t be replaced fairly easily in a kiln, but you don’t want to end up having to replace everything. You’ll most likely need new elements, and they can be unpleasant to change, but anyone with normal agility can do it.

      It’s always possible that some of the switches may be bad or that the kiln sitter might need repair or replacement, but these things are pretty long-lived as a general rule. Unless you have a friend with electrical knowledge, you’ll just have to believe the report of the seller. Hiring an electrician to check it out is likely to cost more than the value of the kiln. New kilns are obviously very expensive, but old kilns, typically, are not. You should shop around a bit online and decide how large a kiln you’d like, and what features you’d like it to have, what the kilns cost new, etc.

      One thing you’ll really want to have is a good track for the elements to run in. If the element holders are all broken up (as they can often be), you’ll have to hold your elements in with pins, and that’s not a lot of fun. The lid should fit smoothly without gaps. Once you get the kiln set up and firing, you might find that the lid lifts a bit and you see an orange ring around it–some kilns are built like this–but when the kiln is cold, the lid should fit well. That’s all I can think of at the moment. It’s really a matter of knowing what you want and then assessing the individual kiln. If you’re just starting out, you might be better off with a smaller kiln. They take less time to fill and cost less and are quicker to fire.

      I’m sorry I can’t give you a lot more help than that. Most important, I guess, is to study enough that you know what you need and then wait until you find the right thing. Oh yes, you really need to assess your situation as to where you’ll set up the kiln, etc. I think I have a post on here about safety and setting up the kiln–you might want to check that out. It’s important that your power supply be adequate to fire your kiln and that the kiln be close to the power source. You’ll need the proper-sized circuit and you’ll need to make provisions to vent the kiln and provide proper base and setbacks, etc. If you have some other specific questions, please feel free to ask, and I hope this helps.

      God bless, Cindy

  4. Beth January 23, 2009 at 11:01 pm #

    Thank you, Cindy!

    God’s blessings on you too!
    Beth

  5. junkartist February 24, 2009 at 10:26 am #

    I need to read all of this, I’ve been working with glass but just discovered ceramic decals so thinking I might try ceramics. It all seems very daunting at the moment though. Love your ceramics!

  6. cindyinsd February 26, 2009 at 4:18 am #

    Hi, Junkartist

    (Love the name!) 🙂 I haven’t worked with decals at all, but my understanding has been that they’re pretty simple to fire. It’s a low-temp (relatively) operation and your supplier should be able to give you some easy directions. You do need to keep in mind the venting, though, as many low-temp firing options still release toxic fumes.

    Best of luck, and God bless,

    Cindy

  7. Jacqueline June 18, 2009 at 2:19 am #

    How generous of you to share your knowledge. I am reading and rereading your posts on firing. I recently bought a kiln with a kiln sitter and I am still trying to get the hang of it. I am wondering if I should spring for a controller. Thanks so much and love your work!

  8. cindyinsd June 20, 2009 at 4:49 am #

    Thanks, Jacqueline. 🙂

    The controller is a great thing to have, and if you’re going to do a lot of crystalline matte glazes you’ll likely want one eventually, but you can do some great firing with just the kiln sitter. I hope your new project goes super for you!

    God bless, Cindy

  9. Jacqueline October 18, 2009 at 1:50 pm #

    Excellant info. Thank you very much. This should be in a magazine! Thanks again,
    Jacqueline

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: