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Troubleshooting
BISQUE:
How can I tell if my bisque has been fired properly?
This is very difficult for the studio to do. You can do absorption
tests and test for coefficient of thermal expansion, but these tests can
be complicated and inconclusive. The best way to assure your self of quality
bisque is to use reputable bisque suppliers with a history of good quality.
If you are just starting out or having problems with certain bisque, you
may want to meet with the supplier to make sure they have a history of
good quality and active testing program on all the industry's leading
color lines. Work with those who give you consistent quality and service.
There are many quality suppliers out there who are willing to work with
you on this issue.
How can I tell if my bisque has a small crack that may not be visible?
Do the "ping" test. Hold the piece of bisque loosely in
your hand and tap the edge with a pencil. It should ring like a bell.
If it sounds like a clunk, the bisque may have a hairline crack.
GLAZES:
Crazing:
Crazing is a network of fine hairline cracks that appear on the glaze
surface. If the glaze, during the cooling process, shrinks more than the
clay body you get crazing. There are two types of crazing, immediate and
delayed. If you get delayed crazing, weeks or months later, it is probably
caused by moisture getting into the ware.
Some causes of crazing are:
- Under fired bisque.
- Opening the kiln too soon before it has cooled properly. Do not open
the kiln until it is below 150°F or is cool to the touch.
- Incompatibility between bisque and glaze.
- Moisture can get into the piece through stilt marks and the water
expansion contraction caused by heating and cooling of ware, i.e.: repeated
microwave or dishwasher use can cause this over time.
Pinholes and Craters:
What are pinholes and craters?
Pinholes and craters are two of the most common glaze surface defects.
After firing, you may see small "pin sized" holes on the surface
of the ware (pinholes) or you may see larger "crater like" impressions
like the surface of the moon (craters).
While there are many things that can cause pinholes and craters, the
causes are related to gases coming from either the clay body or the glazes
that have not smoothed over after firing.
The clay body, and even the glaze, contains clays and other minerals
mined from the earth. These contain some organic matter (i.e. plants,
etc.), carbon and more. These materials form gases during the firing process
and can result in pinholes and craters.
Several things can cause such defects:
- Heavy glaze application
- Fast firing and/or underfiring of the bisque. This does not give enough
time for the organics, etc. to burn out.
- Fast firing or underfiring of the glazes. Does not give enough time
for the glazes to smooth over the surface of
the bisque.
- Not enough oxygen in kiln for carbon to properly burn out.
There are many possible causes of this problem and it can be tricky to
properly diagnose without a lot of help from the studio concerning the
bisque used, glazes used, firing schedule, cones used, where piece/s was/were
in the kiln, etc.
We believe the following two ideas are important to reducing and/or eliminating
the occurrence of pinholes and craters.
First, we have seen far greater incidence of these issues with fast firings
or firings that go too fast toward the end of the firing because the glazes
do not have time to smooth over any potential pinholes or craters. So,
by
maintaining a proper firing schedule, you should be able to greatly reduce
or eliminate these surface defects.
Second, we find there is also a much greater incidence of these glaze
surface defects with a heavy color or clear glaze application. By making
sure the application is appropriate, you can also help eliminate these
issues.
Can I Fix the Piece for My Customer?
Finally, here are some suggestions if you want to try to fix a piece that
has either pinholes or craters. If the issue is the glaze firing (i.e.
it might be under fired or fast fired), you may want to re-fire to the
proper schedule. If the problem may have occurred in the bisque firing,
try grinding down any of the thick craters with a dremel tool, brush on
a thin coat of clear glaze, and re-fire to the proper cone 06 schedule.
This may not always work, but it is the best option. If the glaze has
been applied too heavy, a re-firing will probably not work.
Small spots where the clear glaze seems to "crawl" away from
the bisque and "bead up, especially on the inside bottoms of mugs,
vases, etc:
There are a few possible answers to this question:
- Over application of clear glaze. This often happens on mugs, vases,
etc. because it is a likely spot for the clear glaze to pool after dipping.
The clear glaze tends to collect at the bottom. The remedy is to make
sure the excess clear glaze is swirled around the inside of the piece
and properly poured out.
- More than likely though, dust or debris has collected at the bottom
of the piece (or oils or lotions from hands on the surface of other
pieces). The best remedy is to make sure that the bisque is properly
cleaned with a damp sponge prior to painting including the inside bottom
of pieces.
Peeling of glaze, shivering or popping off:
What is shivering, what causes it and what can I do to limit it in
the future?
When you fire bisque in the glaze firing, the bisque will basically
expand during the firing and contract during the cooling phase. Shivering
is typically defined as occurring when the bisque and glaze shrink at
different rates. If the bisque shrinks more than the glaze, the glaze
has nowhere to go but "shiver" off or "flake" off
the bisque. "Shivering" is normally defined as the opposite
problem to "crazing" since crazing is often a result of the
glaze shrinking more than the bisque. The result is a network of fine
cracks in the glaze.
Unfortunately, shivering happens with almost all studios over time no
matter whose colors or bisque you use. Our goal is to help educate studio
owners and managers to the potential causes so the affects of shivering
can be limited.
Potential Causes
What are the potential causes of shivering? The following is a list
of key potential causes:
- Underfired bisque (and on some occasions over fired bisque).
- Clay or slip used in the making of the bisque does not fit the glazes.
There can be multiple causes for this problem. Clay is mined from the
earth and this is an inexact science. Sometimes there can be issues
with the clay that cannot be detected until you do a glaze firing. This
happens infrequently. Also, bisque suppliers are continuously getting
their bisque from multiple suppliers from different parts of the world.
This means there are more types of bisque being introduced into the
market place all the time. Make sure you are using reputable bisque
suppliers and they have a history of consistent quality. We should emphasize
that even reputable bisque suppliers may, as they develop additional
sources, deliver bisque that shivers.
- Heavy color application.
- Not allowing the colors or clear glazes to dry enough before firing.
When heated, moisture from the color can turn to steamand cause a poor
fit between color and bisque. We feel this is a much bigger problem
in the busy season when both customers and studios are crunched for
time.
- There is a theory that using hairdryers on a hot setting can increase
the potential for shivering.
- Cast bisque being cleaned too much around the edges.
- Pressed bisque being compacted too much on certain areas of the pieces
like the edges.
- Sometimes, bisque, when shipped, can rub against the box and create
a polished surface that does not allow the color to adhere.
- Oils, dust and soluble salts on the bisque can cause shivering because
they do not permit the glaze to bond properly with the bisque.
- Any combination of the previously mentioned points.
As you can see from the large list of potential causes, it can be quite
difficult to diagnose the cause of the problem. To help diagnose almost
any ceramic issue, we recommend keeping a thorough log of problems which
includes such things as the type of bisque and color used, application
of color, firing history, where on the piece the shivering occurred, etc.
This log will help you organize facts, to present to your supplier who
will then be in a better position to help determine what may have caused
the problem. Without a good log, finding a solution will be almost impossible.
To limit shivering in the future, we recommend some of the following
remedies:
- Use bisque suppliers with a history of good quality
- Make sure that the application of color is not too heavy.
- Try to limit "rushing" the ceramic process by either firing
too fast, using hairdryers on a hot setting, or firing potentially wet
color orglaze.
- Make sure customers hands are clean and the bisque is wiped down with
a damp sponge before painting so any dust or oils from lotions or food
have been cleaned from the surface.
Glaze/Underglazes compatibility:
Without testing each and every manufacturer's products with each possible
combination, it is impossible to guarantee. But most colors, if not all,
should be compatible with each other. The best way to assure you is to
do some test tiles.
Running Glazes:
When this occurs it will likely be a darker color like black, cobalt blue
or dark green.
The most common causes are:
- Thick application of color.
- Thick application of glaze.
- Not allowing the piece to dry thoroughly before glazing.
- Glaze firing too hot.
- Combination of the above.
The interesting aspect of this is that lighter colors do this too but
they are often not noticeable to the naked eye because the pigment is
not as strong.
Hazy or milky colors:
The main causes of hazy or milky colors, which are normally tiny bubbles
trapped in the clear glaze, are a heavy glaze application or a glaze firing
which is too cool. If it is the latter, this may be fixed by refiring
to the proper cone value. Use shelf cones throughout your kiln to verify
your kiln is firing to the proper cone value. By re-firing you are allowing
more time for the bubbles to work their way up through the glaze.
Grainy or Rough Glaze Surface:
If you are getting a grainy, rough or matte surface on your clear
glaze, it is often caused by either a thin application of clear glaze
or an underfiring of the glaze.
Mixing a non-toxic glaze and a toxic glaze together:
Toxic and non-toxic formulas can be very different so we do not suggest
you try this. However, with so many different brands and formulas, contact
your manufacturer for the definitive answer. Remember that in mixing a
non-toxic glaze with a leaded one you will lose the non-toxic properties,
and you will probably make the piece no longer dinnerware or food safe.
Mixing underglazes together to get different colors:
The answer with almost all like brands is yes. For example, if you
want to get a lighter blue you can normally mix a little white into dark
blue. This can be very helpful for your customers in decorating. You may
want to do some test tiles with your colors for your customers and contact
your manufacturer for their information.
We do recommend however, staying within one manufacturer's product when
you mix. There is no way to insure compatibility between manufacturers.
Double dipping pieces:
Here are two techniques to remedy the situation. One is to use tongs
and single dip and simply touch up with a brush (wear gloves). Another
way is to double dip, but dip it in a way such that the line is in an
inconspicuous spot. For example, if you are dipping a plate, dip at least
3/4 of the plate such that you are double dipping a small section of the
plate. This will not be as noticeable as dipping a little more than half
the plate so the line is in the middle.
Changing dipping glazes, from leaded to non-leaded or from one manufacturer
to another. Can what is left in the dipping tank be combined with the
new glaze added:
We do not recommend the practice of mixing unlike glazes. Different suppliers'
products may not be compatible with one another. You could get inconsistent
finishes, blistering, or other problems. If you are mixing leaded glazes
that have been individually tested to be dinnerware or food safe, you
cannot be assured that they remain food safe after they are mixed. It
is even more risky if you mix leaded and non-leaded glazes together. You
would be mixing a moving and non-moving glaze together and they may not
work at all, and you probably will be destroying their food safe properties.
Repackaging paints and putting them in your own jars:
Not without understanding and adhering to federal and state labeling
laws. Through the American Society For Testing And Materials (ASTM), you
can obtain what is called the, "Standard Practice For Labeling Art
Materials For Chronic Health Hazards D 4236."
This five page "standard" basically lets both the producer
and repackager know what their responsibilities are in labeling the color
properly. If you are repackaging your paints, we suggest you send for
the standards and have your lawyer advise you on this matter.
American Society for testing and materials
1916 Race Street
Philadelphia, PA 19103
Website: http://astm.org/
FIRING:
Small debris on ceramic ware after firing:
There are several possibilities:
- House keeping- both inside and outside of kiln
- Bits of firebrick and flakes from heating elements that have loosened
or fallen. Due to the extreme heat during the firing of the kiln, a
kind of convection effect is created and debris can blow throughout
the kiln and deposit onto your ware. The solution is to vacuum kiln
before each firing or as necessary.
- Dipping glaze may have become contaminated from any of a number of
sources. We recommend using a 100 mesh screen sieve whenever the glaze
looks contaminated. Keep the glaze container tightly covered when not
in use.
- Ware stored after dipping may have debris fall on it from heating
ducts, abraded aluminum trays or shelves or other contaminants.
Large plates and platters split or cracking during the firing:
- This could be in the making of the bisque or it could be in the glaze
firing. Make sure that the plate or platter is raised properly off the
shelf with stilts and there is proper kiln ventilation (a venting system
is highly recommended). This will allow the ware to heat and cool at
an even temperature. Placing the large platters as close to the middle
of the kiln when loading will also help. Do not place on bottom or top
of kiln or near peep holes where cool air can cause thermal shock.
- The problem could have been caused before you even received the plate.
For example, a stress crack may have developed in the making of the
bisque or the bisque may not have been properly fired. Some bisque manufacturers
stack their plates and/or do not give proper ventilation to the larger
items and the entire piece does not get the same "heatwork".
This could lead to cracking during the glaze firing.
- Firing and/or cooling too fast
Yellowish spots on the bisque after the clear glaze firing:
These are most likely "hard spots" which occur in the pouring
of the greenware. If the slip hits the same spot during the greenware
pouring process, it can sometimes concentrate and pack the clay in an
area thus creating a "hard spot." This may appear as a yellowish
spot on the bisque. However, it is not always present on the bisque and
sometimes appears only after the glaze firing.
The hard spot on the bisque is sometimes difficult to paint but you can
hide the yellow by covering it with white underglaze.
Cause and the cure for the tiny air bubbles trapped under the surface
of the clear glaze:
The tiny air bubbles trapped under the surface of the glaze are gases.
They can occur for many reasons, but most likely, an over application
of clear glaze or an under firing of the glaze. If you do not allow the
organic materials either remaining in the bisque or in the glazes to escape,
it can result in tiny bubbles trapped in the glaze. If you see more bubbles
in the bottom of a mug for example, chances are this is an over application,
because that is where clear glaze tends to "puddle." The remedy
is to make sure your clear glaze is the proper viscosity and your dipping
technique is good. For an immediate solution, you can try to fire it again
but you will get varying degrees of success depending on the cause. Make
sure the glaze is properly mixed.
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