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Question: I had a piece blow up that had already been fired and was in for the glaze firing...it blew and ruined other pieces.

Possible Causes:
1. This sometimes happens when re-firing a piece that was previously glaze fired because the first glaze seals the surface. To be safe, pre-dry it with heat before trying to re-fire a re-glazed piece.

2. Once in a while a student glazes a dry piece of greenware by mistake and puts it with pieces to be glaze fired. This can be trouble if you fire it like a pre-fired ware.

3. I suppose a thick bisque piece that is still full of glazing water could explode if fired fast before it has a chance to dry. This is very rare.

Possible Solution:

1. Preheat your pieces at a low temperature (200 - 300 degrees) for at least 6 hours to remove any possible moisture that may remain in the piece.

HARD TO APPLY
Yes, it can be done. The application of ordinary glazes is challenge because the piece is not porous like bisqueware. Experiment. You may find a good way to do it.

FIRING
Anytime we re-fire a pot we take a small risk that the pot will explode when the moisture in the body of the piece turns to steam. Take it slow (like with greenware) while it passes through the steam forming temps.

AVOID MOST OVERGLAZES
Pottery is often decorated using overglazes, china paints, and lusters. However, to make them easy to brush on, many of these materials are suspended in TOXIC aromatic hydrocarbons. I use lusters, but only when I can sit outside with the wind from my back to carry the vapors away. If you can smell it, your brain is being effected. It is not safe to use. Firing also releases the toxins.

This site is written for adult china painters.
http://www.porcelainpainters.com/solvents2.html

I would not use materials suspended in aromatic hydrocarbons in a school setting.

MAYBE A SAFE ALTERNATIVE
There are water base overglazes as well.
I have not used them, and cannot vouch for the safety. Perhaps other teachers are familiar with them, and can vouch for them. You could call or email Amaco to verify that these are safe to use in a school setting.
http://www.buyamaco.com/acatalog/Opalescent_Versa_Color.html

Marvin

I stand corrected. Not all firing odors are toxic. I was referring to aromatic hydrocarbons used as organic solvents (in china paints and lusters). Some other chemicals can also produce toxic vapors - some without any noticeable odor. I think the companies that sell school supplies are careful not to use hazardous materials (or at least include warnings if they do), but we need be vigilant. I am not familiar with "concept" underglaze and could not say if the underglaze you used has anything that is both volatile and toxic in it. Perhaps you should ask the company that sells the product to tell you the source of the firing odor. Tell them at what firing temperature the odor is worst, and try to describe nature of the smell. Also be sure you do not get the same smell from the clay without the underglaze on it. Many clay bodies contain enough sulphur to make a significant haze in the room toward the end of the firing if not well exhausted. You can even taste it. This may not be as harmful as some fumes, but I certainly would not want to breath much of it. I have seen glass window panes above kilns that are etched by the kiln acid fumes so badly that they could be used for bathroom privacy glass.

The envirovent systems and other vent systems need to be routinely checked before and during each firing and repaired if any odors are noticed.

1. Look and be sure the openings are not clogged where the system draws fumes from the kiln. Have a piece of coat hanger wire handy to check before loading the kiln to see that the hole(s) is (are) open in the kiln bottom.

2. Be sure the lid and the kiln seams are nice and snug. If you see gaps on an old kiln, seal it with some kiln fiber insulation or at least stuff some clay in the openings. You make wadding with clay and sand that can be used to seal a hot kiln.

3. Be sure peep holes and thermocouple holes are tightly stoppered. Check under the kiln during firing to see if it is sealed tight against the kiln when the kiln is hot. Heat warps the kiln and can leave a gap. Some newer vents are spring loaded to keep pushing them up against the kiln bottom.

To test an under the kiln vent system, close the kiln and close all the peep holes. Close the top peep hole with some clay so it leave a half inch hole. Hold a burning match near this hole. If the flame is drawn toward the hole, it tells you that the vent fan is creating a negative pressure in the kiln and it will be drawing fumes out. If this works on a cold kiln and not on a hot kiln, the seal my be bad at the bottom between the kiln and the vent when the kiln warps from heat.

4. Finally, check the outgoing tubing to be sure there are no leaks at the joints or otherwise. If you find any breaks or leaky joints, seal them with high temp silicone caulking or glue. I was called to one very smelly school in mid winter. The roof vent was covered with snow and the kiln odor was blowing into the space above the ceiling tile and passing on to the hallway and adjoining rooms in the building.

Many teachers have the kiln finish off at night so the air is clear by morning. If it goes off about two hours before you come in the morning, the residue should be clear if you have any kind of exhaust working. Always check first thing in the morning to be sure it went off. I have seen too many things go wrong with automatic kiln controls to tell all the stories here. If firing on Friday night, it means a phone call or a trip in to check it on Saturday morning -- not Monday.

Marvin (firing kilns for 47 years and still breathing fine)

>My collegue at the high school suggested I try his method of waxing the bottoms of pots that were going to be dipped in glaze. It worked wonderfully well, made the dipping and shelving of glazed pots so simple and easy. No stilting needed. It was really fast, and I love fast. . . .
>
> Except .... firing it is very smelly. . . .

I never fire with stilts and seldom use wax. Some high fire clay sags if fired on stilts. In the past, I have used wax on the bottoms of pots. I find that it is just as effective to 'dryfoot' the pot and sculpture bottoms by sponging the bottoms after glazing. I decided that the wax costs about as much as the wasted glaze that is sponged off. I also figured that it is probably an environmental wash between smoking up the air with petroleum products or washing a little glaze down the drain. If students use hot wax, there are safety issues. If wax resist is used, the cost of the wax is higher.

It is possible, if you can deal with the hassle, to be save the glaze by sponging the bottoms into a water bucket and letting the glaze settle to the bottom. Pour off the water, and keep the glaze ingredients to use as a 'hash glaze'. I call it 'wonder glaze". Some call these 'Glaze X', Y, and Z. After it accumulates, test it to see what color it is, and it becomes another color choice. If nobody likes it, add a bit of colorant like iron oxide and test it. If you are using all non-toxic glazes, an ugly hash glaze color that fires smooth can be used for the inside of closed forms as cheap liner.

With either system, whoever loads the kiln has to check every bottom. With wax there is always the chance that a student left some glaze beads on the wax. When sponging off the glaze, there is always the chance that a student forgot to do it, or did it poorly.

We always have to decide what to do with the work of careless students. If there is more work than fits in the kiln, I set it back and do not fire it until the student cleans it. If there is room in the kiln, I sometimes fire it on a little bisque slab with thick kiln wash and let forgetful student chip it off. They can glue felt on the bottom to conceal the mess. We can stilt it or clean it up for the student and ignore the oversight (not educational, but sometimes it is expedient).

Marvin

>I fired projects last semester with no problems. I have an electric kiln that I use Amaco Glazes. We apply 2-3 layers of glaze that come out glossy. I fire them at cone 5. The last few firings the glazes are discolored and have a matte finish. I have looked into it some - and it says that it is underfiring the projects. Does anyone have any advice of what I should change on the kiln or if I need to do something different. Any help would be appreciated.
>
> Thank you!
>
> Angela

Here are three ways to produce matte glaze surfaces.

If the glaze did not get hot enough to melt completely, it may feel a bit like fine sandpaper.

Try setting a piece so it can be seen when looking in through the peep hole. When the kiln reaches temp the glazes should all look very glossy if they are thick enough and if they are hot enough to melt.

Some glazes are formulated to form a microscopic crystalline surface during cooling, but when at peak temperature they too are glossy. For these glazes, slower cooling allows for a more matt surface than faster cooling. These feel smooth, but have a softer look than glossy or under fired glazes. They can be very nice on beautiful pottery forms and on sculpture. I have seen some exquisite vessels with matte glaze decorated by brushing or trailing a motif or design on it using another color or a clear gloss glaze.

Sometimes a thin application fails to cover all the texture of the clay, so it is more matte looking.

Marvin Bartel, Ed.D.

>Hi-
>
> I have an idea for a clay research project to do with my high school students. I want to generate a list of commonly asked ceramics questions ( I need at least 30-35 questions) ranging from all aspects of making clay sculptures to using the pottery wheel. I want them to find the answers on the Internet and then create an "answers" poster for their questions that we can then display in class. Of course I am under the gun as I go to my conference this coming Thursday so I need to get this organized. Does anyone have something similar or have some ideas of questions that i can have the kids answer?
>
> Thanks for the help!
>
> Kim

Here is a bunch of questions. Some may work for you.

How are your arms and body positioned to facilitate easy centering while throwing?
An answer is at: http://www.goshen.edu/~marvinpb/throw/center.html
Why does clay warp and crack when it dries?
An answer is at: http://www.goshen.edu/~marvinpb/dryingslabs.html
How do you fix a glaze that tends to run off of the pot during firing.
Answer: add some clay to it. The amount needs to be determined by experimentation.
What is the difference between expressive pottery and abstract expressionism in clay?
Who is the most famous woman clay artist who lived and worked in the last 200 years?
How does salt-glazing work?
How can the same glaze be red in some firings and be green in other firings?
Hint: This glaze contains some copper.
Why does clay explode in the kiln?
When was the wheel first used for making pottery?
What are common ways to fire pottery without a kiln?
How is pottery finished without glazing it.
How is clay that you dig yourself processed and prepared for using?
An answer at: http://www.goshen.edu/~marvinpb/dryingslabs.html
Who was the longest living working clay artist in the United States and what was she famous for?
Hint: She died in 1998 at 105. Her book is titled, I SHOCK MYSELF.
How is slip for slip casting different than slip that collects in the water while you are throwing a pot?
Hint: How and why is the acidity different?
What are three or more advantages to having holes in some bricks?
Why would a sculpture that consists of a cube balancing on one corner be a sculpture in the round.
Who was Viola Frey? Why was she important? What was the style of her work?
Who was Shoji Hamada? Why was he important? What was the style of his work?
Who was Benard Leach? Why was he important? What was the style of his work?
Who was Peter Voulkos? Why was he important? What was the style of his work?
Who is Ruth Duckworth? Why is she important? What is the style of her work?
Who is Toshiko Takaezu? Why is she important? What is the style of her work?
Who was Adelaide Alsop Robineau? Why was she important? What was the style of her work?
What is the chemical difference between the unfired clay and the fired clay molecule?
Why have people died from breathing ordinary clay dust?
What temperature is needed to convert clay to bisqueware??
What are the advantages of single firing compared to the advantages of using a two-firing process?
Why do some glazes craze?
How does cone 6 differ from cone 06?

>So here's my question -- if a student has a piece that has already
> >glaze-fired, and she forgot to use the clear glaze on top, can she
> >re-glaze it and then glaze fire it again?
> >
> >Thanks
> >Becky
>
> In most cases this works fine.
>
> If the glaze firing has left the clay harder and less porous, you have to try harder to get enough glaze thickness. If this is a problem, use a thicker glaze mix or heat the piece a bit for glazing. I thicken glaze by pouring off the top water before stirring it (then pour the same water back in when finished). In rare cases where the piece has been fired to stoneware temperatures such as cone 6, the piece explodes if it is re-glazed and fired before all the glazing moisture has been dried out. This is probably not a problem for the cone 06, 05, and 04 glaze firings.
>
> Marvin

> >>>Could anyone tell me why my students wheel thrown pots will get cracks
> >>> in the bottom of their ware?

There are several standard precautions to prevent the shrinkage cracks (also called S-cracks by potters) in the bottoms of thrown pots.

1. After opening and shaping the interior bottom, compress the clay by moving back and forth over the clay to lay the clay particles flat making a stronger bottom.

2. Be sure to remove all extra water or wetness from the inside of the pot. Water that remains will retard the drying of the bottom. Late drying shrinks the clay after the walls are too dry to shrink with the bottom.

3. Make the bottom thickness from 3/16 to 1/4 inch thick. If it is too thick it will dry too slowly. If it is too thin it will be too fragile. When opening, we use a pin tool to puncture the bottom to measure the clay thickness. For a cheap pin tool, you can poke a hat pin through a cork.

4. Take the wet pot from the wheel to a porous bat that will draw out some moisture. I use dry unglazed ceramic tile (commercial quarry tile) rather than plaster bats because plaster chips are a pain the rework clay--causing too many broken pieces from pop-outs when bisque fired.

See this illustrated guide for how to remove wet pots from the wheel.
http://www.goshen.edu/~marvinpb/throw/remove.html

5. As soon as the top is strong enough, invert the pot so the bottom dries first or with the rest of the piece.

Marvin Bartel
Dr. Marvin Bartel, Ed.D., Professor of Art Emeritus Goshen College

CONE SITTER PROBLEMS
Generally, this happens because the previous firings have produced some fumes that have been escaping through the small openings around the rod that lays on top of the cone. As the fumes exit, there is some condensation of the volatile chemicals that are in the fumes that escape from the kiln. As the vapors condense, they eventually gum up the mechanism and the rod gets stuck. Hence, it does not move as the cone melts. It does not release the weight that has to drop down to turn off the kiln.

Some variation in firing can happen if the cone is not centered in the sitter, but it generally does turn off eventually. There is a little plate on the outside that can be adjusted (two screws) to make sure the trigger hook operates without friction (unless it gums up). There is a metal trigger piece in the weight that slides up and down (one small set screw tightens it) to adjust the amount of movement needed to release the hook as the cone melts. You can set to require less cone movement.

ROUTINES THAT HELP
Every time I insert the cone, I first wiggle the rod to be sure it feels free and easy. I also check that is centered between the cone supports. It generally takes quite a few firings before the mechanism gets gummed up.

PREVENTING THE GUM UP
If the kiln produces fumes, installing a bottom mounted vent will help. This creates a vacuum in the kiln so fumes do not exit though the mechanism.

In some kilns, I have to place the sitter lower in the kiln. This helps keep it clear. The chimney effect tends to make any small openings in the top portion exits for fumes. The bottom openings tend to be entrances for room air.

SAFE PRACTICES
Computerized kiln controls are better, but a person must still manually check the kiln shortly after it is expected to switch off. I do not fully trust any un-redundant system or person. When I fire, I place a small piece of masking tape on my wristwatch with the letter K on it. When students fire, they have to phone me when the kiln is off so I can remove my piece of tape. It saves the kiln while allowing others to learn how to do it.

Always include at least one witness cone (a cone you can visually check in front of one of the peeps). I set it in clay that includes a spoon shape to catch the bent cone. Keep the cone clay quite thin so it does not explode in a glaze firing. If the clay is wet going in, I poke many pin holes in it to vent the steam from the thick parts.

Marvin

Marvin Bartel, Ed.D., Professor of Art Emeritus Goshen College

When clay is painted, we call it room temperature glazing.

One method uses acrylic as a base coat. Let it dry, then put on a contrasting tone and wipe it off before it dries to bring out the texture.

Bisque or painted bisque can be waxed. Paste waxes (sold for wood) can be colored with old crayons, but the wax color preparation needs to be done by an adult. For safety to avoid blowups and serious burns, waxes have to be just barely melted with thermostatic control as in slow cooker, or in an improvised double boiler. It is very dangerous to overheat waxes. 200 F should be the limit.

Once colored waxes are prepared, paste waxes can be used at room temperature. These methods need lots of old rags to wipe and polish the pots and sculptures. Old bristle brushes or cheap throw away brushes can get color into the crevices of the bisque ware.

Laundry detergent and boiling hot water will clean wax out of the old brushes.

Our students often make bisque ware stain for sculptures using iron oxide powder and water as a cheap stain. They wipe it off with a damp sponge to bring out the texture. Then they seal it with clear wax or acrylic medium thinned and brushed on. Clear spray (used outdoors for air safety) is another sealer (used after the stained piece is dry.

Marvin

Marvin Bartel, Ed.D., Professor of Art Emeritus Goshen College

>Okay, I have some tiles that students made that are going to be put
> onto metal posts, outdoors. I have the correct adhesive, but I want to
> know if I need to seal the tiles or just grout them? They will be in
> the weather, but can be covered up during the freezing temps.
>
> Thanks
> Becky

It depends on the tile. High-fire tile is better outdoors in freezing climates because high-fire clay is less apt to absorb moisture. Low-fire tile absorbs moisture in the rain. High-fire is cone 6 to 10 (stoneware and porcelain). Low-fire is cone 010 to 04 (earthenware). If it has not had a chance to dry out after a rain before it freezes, the freezing can break or even pulverize the clay because water expands when it freezes. Using a waterproof sealer could help if it is totally dry prior to application of the sealer. The grout for any outdoor tile should be sealed after it has dried a few weeks. If they are low fire tile, perhaps the whole tile could be soaked in sealer and allowed to dry before installation.

Marvin

KILN SAFETY
Most electric kilns are UL Approved. Kilns will not cause a building fire if properly installed. The floor under the kiln should be something like concrete, masonry, or brick. Kilns come with instructions about wall clearance requirements that need to be followed. As it overheats, the kiln elements eventually burn out. When the elements burn out the electrical circuit is broken and the kiln begins to cool down.

Nothing flammable should be placed or stored, even temporarily, in the kiln area. I have seen frightening situations where art teachers had thoughtlessly stored combustible materials too casually and too close to the kiln area.

KILN DAMAGE
Of course the artwork, if made from low fire clay, will melt and possibly damage some of the kiln bricks. If the artwork is made from high fire clay, the clay may still deform and glazes will likely still run off if the kiln fires too long.

HOW TO IMPROVE EDUCATION
When presenting a rationale about an art facilities and equipment, I generally point out that our choices need to be based on what is best for the education of our students. We do not build a gym for the coach. We provide safe facilities and equipment because of the benefits to the students. Teachers, administrators, school maintenance workers, custodians, and outside contractors like insurance companies sometimes need to be reminded about the mission of a school.

The health and safety of the children and workers is the first concern. Education is second. Preservation of property is very important, but would probably be third on the list. If we select an insurance company that puts too many limits the education of our children, it may be time to get three bids on insurance--not merely based on cost, but based on cost AND benefits.

FIRING OPTIONS
If a kiln is safely installed and nothing combustible is nearby, it seems safe to fire during the night if somebody checks it the morning after. Night firing is commonly done to avoid breathing the fumes when the vent system is not quite adequate.

If finishing the firing after work is not allowed, it may be possible to set the kiln on a low or slow setting overnight during to pre-heat it so the firing can be rapidly finished during the school day.

If this is not possible, it may work to use a two day cycle. Use the first day to pre-fire the kiln a low setting to dry out the artwork thoroughly. Turn it off for night. On the second day, do a fast firing so it finishes off during the school day.

FAIL-SAFE BACKUP ALTERNATIVES
If an insurance company proposes a costly fail-safe operation that takes control away from the teacher, perhaps they would also be willing to look at alternatives that are less costly to install. It may not cost as much to install a heat sensor that would cut the power to the kiln if the room temperature gets above a preset point. A single sprinkler head installation in the kiln area may also be a less costly alternative to taking the control away from the teacher.

Handbuilt famous buildings (i.e. St. Patrick's cathedral with rose windows)

Combination of handbuilding and wheel thrown (or just sometimes only
handbuilt) pots with clay plants (I used a plant book for authenticity, and we made copper stripped "cookie cutter type things" for leaves based on the book, to cut out leaves from the slabs, and coils for the stems...these are always great at any ceramics level).

Clay apartments (each kid gets a certain size, like a shoebox, and exposes the front and 'ceiling') and handbuilds the insides of the apartment, then you hot glue them stacked one atop the other for the apartment.

Thrown goblets with handbuilding that is illuminated letter inspired with zoomorphic designs wraping around and up to the 'cup'.

Thrown t-pot sets, which include thrown lid, pulled handle, and then 5-7 cups, all one theme.(I do organic themes)

Troemp l-oile (sp??) "Leather" handbags that sit open, or jackets that hang on walls, with zippers etc.

Combination of coil pots and coiled baskets. You make the coil pots, then leave clay loops at the top, and then coil onto the loops(after the partial finished work is fired of course) the basket material and continue the pot shapes.

Clay pop up books, which essentially means a clay book, that is "opened"
onto a specific page, with the images that are 3-D popped up like a real pop up book.

San D

If you have creative ideas that have worked well with your students, would you email me? So far I am thinking about graffiti/diversity inspired slab houses (inspired by Reinaldo Sanguino), large flat coil vessels, and a sculpture inspired by a ceramic artist. They will have 6 weeks on the wheel as well and I have some ideas for the wheel, but again would appreciate any creative ideas anyone has. Thanks!

Kelda Van Patten
Hillsboro High School
Hillsboro, Oregon

Your wrote:

>1. I had a bunch of glaze drip off of a project and stick to the kiln
> shelf (I did use stilts). I tried chiseling it off and I cannot get it
> all off. I looked at some posts online and they suggested using a
> power chisel. I do not have access to one. Does anyone have any other
> ideas to save the kiln shelf?
>
> 2. Also, we've been making clay looms. Most of them have turned out
> great, but we had a little problem with some of them breaking, because
> they are thin, flat pieces. Also, the holes sometimes got covered in
> while glazing (we tried poking through them with pins before firing,
> but some of the glaze still ran into it). Has anyone made clay looms
> before and have any suggestions?
>
> Thanks, Marcia

1. _ _ it happens. To remove glaze from shelves, I use a standard freshly sharpened stone chisel with a steel mallet (hammer). The chisel is re-sharpened with an electric grinder (dipping in water very frequently to prevent over heating that removes the hardness). A belt sander is also good for sharpening tools with less overheating than a grinder.

Set the shelf vertical on the floor so it is learning against the wall nearly vertical using a sponge behind it at the top edge (to cushion it a bit). Hold the chisel as vertical as possible so you do not break the shelf when you whack it. Keep whacking at the glaze globs harder and harder until the glaze breaks off. Turn the shelf as needed to hit the glaze from other directions. This may take a divot out of the shelf, but you can fill the divots with thick kiln wash. A hand held high speed electric grinder also works. Anyway you do it, it is a labor intensive job. ALWAYS wear eye protection.

If you cannot or choose not to remove the glaze. Cover it with heavy kiln wash mixed with some silica sand and fire it with the glaze still on the shelf. You can try again to chip it off after another firing. To prevent the problem next time, use more kiln wash on the shelves and less glaze on the pots. Also, be sure the kiln is not firing hotter than it would need to.

2. I was wondering why the clay looms would be glazed? In the Dremel tools section in the hardware store or building supply store they sell small diamond wheel points. These diamond drills can cut through glaze, but it may be better in the future make the pieces without glaze on this area of the piece. This page shows the diamond wheel points.
http://www.dremel.com/en-us/attachments-and-accessories/attachment-accessory-group.htm?H=188537&G=66239

If I want to glaze an area with holes (like a salt shaker), I use wax resist to fill holes prior glazing. If holes are too large for wax, I fill them with wet paper wads prior to glazing. I poke these out and toss them before firing.

Marvin