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Troubleshooting Bisque, Glaze and Firing

About the best way to make sure yourself of the quality of the bisque,
is to use well known bisque suppliers with a history of good quality products.

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Altering a Greenware Piece
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Pottery Bisque

How to tell if a piece of bisque has been fired properly.
It is very hard for the studio to do.
They can do absorption tests and test for heat expansion but these tests can be very hard to do and don't give definite answers.
About the best way to make sure yourself of the quality of the bisque is to use well known bisque suppliers with a history of good quality products.
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 are actively testing all of the ceramic and pottery's leading color lines.
Work with those who give you good reliable quality products and service. There are many very good suppliers out there who are willing to work with you on this matter.

How to tell if the 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.

Warped bisque

Incorrectly removing ceramics from the mold can cause the piece to warp. Ceramics should not be removed from the mold until it is leather hard and releases easily.
This is extremely important in porcelain casting.
Porcelain has a memory and cannot be reshaped.


Placing the ceramics too close to an element can cause one section of the ceramic to mature prematurely and warp.
Keep the ceramic about one inch away from the kiln side walls and kiln sitter and thermocouple if one is installed, to insure proper ventilation.


Improper support on stress areas could cause warping.
Large flat pieces should be supported with either stilts or posts.
Porcelain should be supported with prop or silica sand.

Repair warped bisque

It is impossible to salvage warped bisque.



Pottery Crazing
Pottery Craze

Crazing is a bunch of fine hairline cracks that appear all over 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, months or years later, it can show up and it is caused by moisture getting into the pottery.

Causes of Crazing

Under Fired Bisque

Opening the kiln too soon before it has cooled properly.
Do not open the kiln until it is below 150 degrees Fahrenheit 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 the pottery, repeated microwave or dishwasher use can cause this over time.

Pinholes and Craters


Pottery Craters

Pinholes and craters are two of the most common glaze surface defects.
After firing, there may be small holes on the surface of the pottery, which are called pinholes or there may be larger dish shaped like impressions that look like the surface of the moon, which are called craters.

While there are many things that can cause pinholes and craters, the causes are both 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 glazes contains clays and other minerals mined from the earth.
These contain some plant matter, carbon and more.
These materials form gases during the firing process and can result in pinholes and craters.


Fast firing or under firing of the bisque.
This does not give the pottery enough time for the foreign materials to burn out.

Fast firing or under firing of the glazes.
This 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 figure out, without a lot of help from the shop concerning the bisque, glazes, firing schedule and cones that were used and where the pieces were in the kiln.

The following two paragraphs show what can be done to reduce or eliminate pinholes and craters.

There have been far greater appearances of pinholes and craters 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.
They can be reduced by maintaining a proper firing schedule.

There is also a much greater incidence of these glaze surface defects with a heavy color or clear glaze application.
If you make sure the application is correct, it will certainly help stop pinholes and craters.

Pinholes and Craters Repair

Here are some suggestions if you want to try to fix a piece that has either pinholes or craters.
If the cause is the glaze firing, either under fired or fast fired, you may want to re-fire to the correct time and temperature.
If the problem may have happened 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 correct Cone 06 schedule.
This doesn't always work, but it is the best choice that you have.
If the glaze has been put on too heavy, a re-firing will probably not work.

Heavy Application of Glaze

Small spots where the clear glaze seems to crawl away from the bisque and bead up, especially on the inside bottoms of mugs and vases, because it is a likely spot for the clear glaze to pool after dipping.
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 the piece.
The best remedy is to make sure that the bisque is properly cleaned with alcohol and a damp sponge prior to painting including the inside bottom of the pieces.

Shivering or Popping Off

Pottery Shivering

When bisque is fired in a glaze firing, the bisque will usually expand during the firing and contract when cooling.
Shivering is usually defined as happening when the bisque and glaze shrink at different rates.
If the bisque shrinks more than the glaze, the glaze has nowhere to go but to shiver 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 bunch of fine cracks all over in the glaze.

All pottery or ceramic workers know that shivering happens in almost all shops at one time or another, no matter whose colors or bisque is use.
Most of the glaze manufactures goal is to help educate shop owners and managers to the causes so the affects of shivering can be limited.


Under or Over Fire

Under fired 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, because clay is mined from the earth and is an inaccurate science.
Sometimes there can be issues with the clay that cannot be detected until you do a glaze firing.
This doesn't happen a lot, but does happen because bisque suppliers are always getting their clay from many different suppliers in different parts of the world.
This means there are more types of clay being introduced into the market place all the time.
Make sure you are using a reputable bisque supplier that has a history of consistently good quality.
Even bisque suppliers that have a good history on quality may have over or under fired bisque, as they get more different clays and can deliver bisque that shivers.

Wet Glazes and Hard Spots

Not letting the colors or clear glazes dry enough before firing can cause shivering.
When heated, moisture from the color can turn to steam and cause a poor fit between the glaze and the bisque.
I feel this is a much bigger problem in the busy season when both customers and shops are rushed for time.
It is thought that using hairdryers on a hot setting can increase the potential for shivering or 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 stick.
Oils, dust and soluble salts on the bisque can cause shivering also because they do not permit the glaze to bond properly with the bisque.

As you can see from the list of potential causes, it can be quite difficult to find the cause of the problem.
To help find almost any ceramic problem, it is best to keep a thorough log of problems which includes such things as the type of bisque, color used, application of color, firing history and where on the piece the shivering occurred.
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.

Avoid Shivering and Popping Off

Use bisque suppliers with a history of good quality
Make sure that the application of color is not too heavy
Don't rush the ceramic process by either firing too fast, using hairdryers on a hot setting, or firing potentially wet color or glaze.
Make sure customers hands are clean and the bisque is wiped down with a dampened alcohol sponge before painting so any dust or oils from lotions or food have been cleaned from the surface.

Glaze and Under Glaze

Without testing each and every manufacturer's products with each possible combination, it is impossible to guarantee results.
Most colors, if not all, should be compatible with each other, but the best way to assure a good outcome is to do some test tiles.

Running Glazes

Running Glaze

When this occurs it will likely be the darker colors like black, cobalt blue or dark green.
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 part 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 tiny bubbles trapped in the clear glaze, are a heavy glaze application or a glaze firing which is not hot enough.
If it is too cool of a firing, this may be fixed by re-firing 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 and should solve your problem.

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 under firing of the glaze.

Mixing a Non-toxic Glaze and a Toxic Glaze

Toxic and non-toxic formulas can be very different so do not try this.
With so many different brands and formulas, contact the manufacturer for the correct answer.
Remember that in mixing a non-toxic glaze with a leaded one you will lose the non-toxic properties and will probably make the piece no longer dinnerware or food safe.

Mixing Under Glazes

The answer with almost all like brands is yes.
If you want to get a lighter blue you can usually mix a little white into dark blue.
This can be very helpful for decorating when a different shade of a color is needed.
I learned a long time ago also, keep a log of the amounts of colors used to get certain colors, it makes it easier to mix another batch of that color if needed.
It is best to do some test tiles with the colors and sometimes contacting the manufacturer for their information and suggestions.

Most pottery workers stay with one manufacturer's product when they mix for different colors.
There is no way to insure compatibility between manufacturers.

Dip and Double Dipping

There are two techniques to help get a even amount of glaze on your piece.
One is to use tongs and single dip the whole piece and simply touch up with a brush where the prongs held the piece.
Just make sure your glaze is the proper thickness and the dipping technique is good.
There is a page in Tips on how to use a viscosity cup and check the flowability of your glaze.

Pottery Glaze

Another way is to double dip, but dip it in a way so that the line is in an inconspicuous spot.
If you are dipping a plate, dip at least three quarters of the plate so 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.

Pottery Glaze 2

Changing dipping glazes, from leaded to non-leaded or from one manufacturer to another.

Don't mix 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 sure 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.


Small debris on ceramics and pottery after firing

Good house keeping both inside and outside of kiln is a must.
Bits of firebrick and flakes from heating elements that have loosened or fallen should be vacuumed regularly.
Due to the extreme heat during the firing of the kiln, a kind of windy effect is created and debris can be blown throughout the kiln and deposited onto your pottery.
The solution is to vacuum the kiln before each firing or as necessary.
Dipping glaze can become dirty from any of a number of things.
It is recommend using a 100 mesh screen strainer whenever the glaze looks dirty.
Keep the glaze container tightly covered when not in use.
Pottery stored after dipping may have dirt fall on it from heating ducts, corroded aluminum trays, shelves or other dirty objects it could come in contact with.

Large plates and platters split or crack during the firing

Thermal Shock

Thermal Shock

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 correctly off the shelf with stilts and there is proper kiln ventilation.
This will allow the pottery 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.
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 don't give proper ventilation to the larger items and the entire piece does not get the same heat work.
This could lead to cracking during the glaze firing.

Firing 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 gather and pack the clay in an area which creates a hard spot.
This may appear as a yellowish spot on the bisque, but 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 under glaze.


Heavy Clear Glaze Application

Small spots where the clear glaze seems to crawl away from the bisque and bead up, especially on the inside bottoms of mugs and vases.
This often happens on mugs and vases, because it is a likely spot for the clear glaze to pool after dipping.
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 the piece.
The best remedy is to make sure that the bisque is properly cleaned with alcohol and a damp sponge prior to painting including the inside bottom of the pieces.

There is no such thing as a weed--only a flower in the wrong place

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Have you ever come up with a good idea while working with your handmade pottery and thought that you would like to share it with others? You have? Well, why not send it to us and we will add it to the tips page for all to see.

Handmade pottery can be a very gratifying hobby that produces fun and satisfying results. For many people it's an enjoyable release that is created by working an inanimate mound of clay into a beautiful work of art that you made through your artistic abilities.

The best way of starting out is to take a few lessons from Youtube. You will probably waste quite a bit in materials when you first get started. Figuring out how to truly make handmade pottery correctly and shape into what you want it to be can be quite an ordeal. The different tools that a normal shop will have can be fun to try. You will soon see which ones you like to use the most and then when you are ready you will know which ones to buy.

With the help of the internet, you can now purchase most if not all of your ceramic and pottery tools and supplies online. We are located far from any well supplied dealers and yet working with reliable ceramic and pottery suppliers online has allowed us to recieve most of our orders within a timely manner.

When you get all set up, just enjoy the hobby and have fun at it. Some people get pretty serious and start selling their creations at craft fairs and small stores, but others just like to create items for themselves, relatives, and friends. Whichever kind of handmade pottery you desire to endeavor, enjoy the hobby and have fun doing it.

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