FIRING INFORMATION

Cone Temp. Chart & Firing Glazes

Kiln Repair & service For southern California contact us at 818/782-1500

Cone Temperature Chart Please note: Due to malfunction, kiln firing temps may vary in the different sections of your kiln. All kilns are not created equal. Different cones & bars                                                                         From left to right. Bar, cone, large cone & self supporting or witness cone. For ceramics, kilns are not fired just to a temperature.  They are fired to a "cone" level, which accounts for time as well as temperature.  Think of it as heat absorption rather than just temperature. The middle self-supporting cone is perfectly fired.  The left is over-fired, the right is under-fired. Cone's come in different numbers, each of which corresponds to a heating-rate/temperature-combination which will make that cone deform.  At the beginning of the firing the cone is standing at an 8 degree angle.  A perfectly fire cone will be bent to a 90 degree angle.  If the cone is bent less, the kiln was under fired.  If the cone is bent more, the kiln was over fired. Notice the way the cones are numbered.  The hottest is 10, going downward to 1.  Then as it continues to getting cooler it continues at 01, 02, etc.  So there is a difference between cone 5 and cone 05!

Cone number	Orton Cones Final temp  in degrees F at ramp rate of 27 degrees F/hr	Orton Cones Final temp  in degrees F at ramp rate of 108 degrees F/hr	Orton Cones Final temp  in degrees F at ramp rate of 270 degrees F/hr	In standard firing, cones of the right number are placed around the kiln and are watched.  When the cones fall the kiln is turned off.  This works the same for gas and electric kilns. Electric kilns with electronic controllers (such as the Skutt KilnMaster series, Cress) have a thermocouple which continuously measures the temperature, records it over time, and shuts off the kiln when the appropriate heat absorption has been met.  So if the ramping temperature is fast, the kiln will go to a higher temperature before it turns off than if the ramping temperature is slow (thus allowing the clay to absorb more heat along the way.)  The final temperature is most affected by the rate of temperature increase over the last 300 to 400 degrees of firing. Note:  Think of the 0 in a cone number as meaning "minus".  So 06 is much cooler than 6 because it is like a "minus 6".
10	2284	2345	2381
9	2235	2300	2336
8	2212	2273	2320
7	2194	2262	2295
6	2165	2232	2269
5	2118	2167	2205
4	2086	2142	2161
3	2039	2106	2138
2	2034	2088	2127
1	2028	2079	2109
01	1999	2046	2080
02	1972	2016	2052
03	1960	1987	2019
04	1915	1945	1971
05	1870	1888	1911
06	1798	1828	1855
07	1764	1789	1809
08	1692	1728	1753
09	1665	1688	1706
010	1636	1657	1679
011	1575	1607	1641
012	1549	1582	1620
013	1485	1539	1582
014	1395	1485	1540
015	1382	1456	1504
016	1368	1422	1465
017	1301	1360	1405
018	1267	1252	1283
019	1213	1252	1283
020		1159	1180
021		1112	1143
022		1087	1094

Trouble Shooting

While we would love to be able to prevent all mishaps, understanding the why's and how's they come into being will help you diagnose and hopefully, prevent problems in the future. Like everything else though, a certain amount of failure should be expected. We nor our materials are perfect all the time.

1. Did you get a good fire? The fact that you set your digital controller to cone 06 does not insure that is what your kiln did. We recommend that you use self supporting witness cones in every fire. Witness (shelf) cones are used in a series of three:

• Guide Cone is one cone cooler than the desired temperature.
• Firing Cone is the cone temperature desired.
• Guard Cone is one cone hotter than the desired cone.

Use cones in each fire and on every shelf. If you simply cannot use all three sizes, use the firing cone on every shelf, preferably two. Keep a record of your results and you will discover your kiln’s personality- hot spots, cold spots, etc. By using witness cones and verifying that your kiln fired successfully and the desired cone was reached, one important variable will be eliminated as a problem source.

2. What type of glaze was used? Knowing the type of glaze that was used and how it was applied may give us a clue.

3. What does the surface look like? Holes, bare spots, fine lines? Following is a description of the problem, possible causes and potential solutions:

Bare Spots in glaze surface:
called crawling.

Problem 1:
Often caused by dirt, dust or oil, prohibiting glaze from adhering to the ware.

Solution:
Make sure piece is clean before painting by wiping with damp (not wet) sponge. Make sure hands are clean of oils, hand lotions, etc.

Problem 2 :
Too heavy of an application.

Solution:
Lighten application. Lightly touchup bare spots and re-fire.

 

Small holes in glaze:
called pinholing.

Problem:
Underfired bisque is continuing to mature during glaze firing, causing emission of gasses and interrupting the glaze surface.

Solution:
Bisque should be fired to 03-04 and glazes two cones cooler to prevent ware from reheating and releasing gasses. Sometimes repeating fire can smooth the glaze.

 

Large Pinholes: also called craters, fisheyes or bubbles.

Problem:
caused by gases escaping during firing due to
a. underfired bisque
b. firing glaze while wet
c. ware not fired hot enough or piece was fired/cooled too fast.

Solution:
a. Bisque should be fired to shelf cone 04 or hotter. Two cone difference between bisque fire and glaze fired necessary.
b. Allow glazes to dry 24 hours prior to firing.
c. Fire glazes to shelf cone 06 or recommended cone. Glazes need an even steady ramping of temperatures and cooling. File down craters, reglaze and refire.

 

Glaze rolls back: called flip back

Problem:
a. Glaze was applied too heavily, without sufficient drying time between applications.
b. Piece was fired too wet.
c. Oil or grease on piece

Solution:
a. Allow glaze to dry between coats, apply thinner coats.
b. Allow piece to dry 24 hours before firing.
c. Make sure surface of bisque is clean of dust; wipe down with damp sponge. Sometimes flip backs can be corrected by sanding down, reapplying color and refiring.

 

Fine lines in glaze surface after firing: called crazing

Problem:
Generally caused by glaze and body fit problem. When talking technical, will be described as a coefficient of expansion problem.

Solution:
Use mature bisque. Some glazes, especially non-toxic formulations, are more sensitive to application.

 

After firing, glaze falls off pieces:
called shivering

Problem:
Incompatability between ware and glaze.

Solution:
Coefficient of expansion. Body shrinks at a different rate than the glaze, causing the glaze to fall off. Be very careful as the pieces of glaze can be quite sharp. Some glazes, particularly non-toxic glazes, are not as accommodating of less than perfect bisque.

Gloss glaze is matte

Problem:
Glaze did not reach maturation during firing.

Solution:
Use witness cones to make sure proper fires are achieved. Refire piece.

Piece cracks in kiln: called “a mess”

Problem:
a. Thermal shock or expansion of two or more glazes
b. Too heavy of glaze in bottom of piece.

Solution:
a. Use same type of glaze inside and out when glazing. Recommended to use gloss glaze inside pieces, even when mattes are used on outside.
b. After rolling glaze on inside of piece, invert piece to drain excess glaze.
 

Loading a Kiln for Best Results Loading a kiln for firing is not a simple matter of placing shelves and stacking ware. The more thought and planning that is put into loading, the better the results. Ware and shelf placement, the size of the load, the firing characteristics of the kiln and the type of ware being firing are all important factors. First the Furniture Kiln shelves come in all shapes and sizes. For economy of space, it is best to choose shelves similar in shape and size to your kiln chamber. For instance, use a round or multi-sided shelf in a round or multi-sided kiln. Keep the size small enough so there is at least 1" of space between the shelf edge and the side of the kiln or the Kiln-Sitter. Also allow some room between the top of your ware and the lid of the kiln and leave space for witness cones amongst your ware. Select posts in heights to accommodate the ware you are firing. Leave some room between the kiln shelves for air to flow, for heat transfer and for removal of fumes. Half shelves are very useful to improve air movement in the kiln. Use two side by side with a 1/2" space between them and you don't lose much stacking space. Some kiln manufacturers recommend placing shelves directly on the floor of the kiln. Most suggest using 1" posts to put the bottom up from the cooler floor. This creates an insulating layer much like a storm door. Setters and Stilts Air movement in the kiln is clearly a big consideration - one of the most important when loading a kiln. Ceramics need to heat uniformly to prevent warping and stresses in the ware. Air needs to move around shelves and around individual pieces. Plates and tiles benefit from the use of tile and plate setters or stackers. Shelf style setters allow air to move under the large flat objects so they heat more evenly. Avoid heating large flat objects directly on the cooler shelf. If you are firing decorated tiles or plates, vertical setters economize on space, and sets can be stacked to fit even more. Glazed ware needs to be stilted or dry footed or the melting glaze will stick the ware to the kiln shelf, ruining both. Stilts also provide space for air to move around all sides of the ware. Porcelain and stoneware can not be stilted. The stilts embed into the ware during firing. Instead, use high fire kiln wash or silica sand on the shelf. Use prop to prevent sagging of porcelain. Consider Heat Distribution It is important to evaluate heat flow in your kiln and to make this a consideration in loading. Use pyrometric cones to determine the heating characteristics of your kiln so you know where the hot and cooler places are. Arrange your ware with different sized pieces on the same shelf to allow better heat flow. Don't Overfill Perhaps one of the most important factors in good fired results is enough air to mature the ware - to burn out organics in bisque and develop best colors in glazes. Shelf and ware placement and the use of setters and stilts can all help this, but here are a couple more tips: When stacking bisque, invert bowls and mugs opening to opening instead of nesting- this helps air move around all sides of a piece and prevents black rings and spots in the bottom of ware. Fire bisque lids and bottoms together. To get the best fit for lids, fire them on the piece they match. This will let the two pieces shrink together so you get a good tight fit. Fire all glaze pieces separately. Leave space between ware - don't overfill. There is a temptation to cram as much as possible into the kiln to economize on firing costs. Ware fired too closely together creates firing problems. If you must overfill, fire very slowly and vent adequately. Mix thin and thick-walled pieces together throughout the load don't concentrate them in one area where they are competing for air and heat. Use downdraft venting to move air through the kiln and to remove fumes created during firing.

Firing Handbuilt or Thick Cast Ware   Most pinch pots, coiled or slab built ware generally have thicker walls than their slip cast cousins, although molded pieces may be cast heavily as well. With these types of pieces, the thicker walls create some unique challenges for firing. Basic problems that can occur when firing handbuilt or thick cast ware include cracking (or exploding) and carbon burnout. Because of the thicker walls it is important to fire slower and control heating and cooling during firing. Preparation of the piece is important as well. During forming, stresses within the piece may result in hairline cracks that appear during firing. It takes longer to fully dry a thick piece. Uneven drying can result in warping or cracking. For pieces properly prepared, handled and dried, the next critical step is firing. Firing Issues Is the ware fully dry? Ware that is not adequately dried will crack or explode during the early stages of firing. Water inside the pores of the ware turns to steam, exerting pressure inside the ware. To fully dry a thick walled piece, the ware needs to be warm for more than 12 hours. Am I firing too fast? All bodies expand when heated and shrink when cooled. If the outside wall expands more than the inner wall, stresses occur. If these stresses are large enough, they pull the body apart and cause cracking. A 1" thick wall can have more than a 10° F difference in temperature between the hotter and cooler surfaces. Firings need to be slowed down for thicker wall pieces. Likewise, it is important not to cool too fast. Have I allowed enough time for carbon burnout? It is important to burn out all carbon from the ware before higher temperatures are reached (1200° F or 650° C). It takes time for oxygen to move into the pourous body, react with the carbon and then leave. If carbon remains, many problems can occur. These include problems with colour, glaze fit, strength, blistering and discolouration. Use of a downdraft vent system, combined with slower heating, virtually eliminates carbon related problems. Heating & Cooling Control The best way to control cracking problems during firing is by controlling the rate of heating and cooling for the kiln. During firing, materials that make up the body undergo many changes. Special care must be taken at temperatures below 1500° F (815° C) to heat the body uniformly. Remember, the thicker the wall, the slower the heating should be done. Above 1500° F temperatures can be increased more rapidly because the changes are less likely to causes stress cracks within the ware. What Kind of changes occur? All clays can many minerals contain water which does not leave the body until above 700° F. Organic (carbon) materials need to be oxidized (burned out). Other minerals, such as calcite, break down and give off a carbon dioxide gas. Minerals such as flint (silica) undergo a sudden expansion on heating to 1060° F and contraction during cooling. How can I control my heating? This depends on the controls for the kiln. With switches, leave them on medium settings longer. It should take more than 3 hours to reach red heat and even longer for thick pieces or a heavily loaded kiln. Make sure the kiln is well vented below red heat and closed up completely above red heat. Keep the kiln closed during cooling for 8 hours or until well below red heat. When did cracking occur? Often the crack itself can be examined to determine when it occurred. If the edges are sharp, then it probably occurred during cooling. If the edges are rounded or if the glaze has flowed into the crack, then it occurred during heating. What else can cause cracking? Uneven heating is a primary culprit that causes cracking during firing. Hot and cold spots in the kiln can cause uneven heating of pieces. Use witness cones to diagnose hot and cold spots and then adjust the switching or use a downdraft vent to help even out the heating. Careful loading of the ware in setters and on stilts can also help heat circulate around the piece. Underfired bisque is not as strong and may crack more easily during the glaze firing. Use witness cones to assure a proper firing and prevent underfired bisque. Gas expanding in air pockets which developed in the ware during forming can cause large cracks during firing.

 

Firing Red Glazes

Red glazes are among the liveliest, brightest colors we can use, but unfortunately, red glaze problems are legendary. Many of us simply give up using reds or accept whatever results we can get, including the problems. Common Red Glaze Problems   improper color development- dark bluish or purple cast to the glaze color loss- glaze looks gray, white poor surface texture- a rough matte finish and/or visible surface defects "The Strawberry Effect"- tiny black dots or spots in the fired glaze crazing- a crackled or cracked appearance in the fired glaze Some of these problems relate to the preparation of the piece and application of glaze, but many defects are the result of improper firing practices. Preparation and Application   ware must be clean and free of dust do not apply red glaze to greenware apply only to properly fired bisque (use witness cones to verify firing) work area and tools should be kept clean and free of contamination no eating/smoking in glazing area glaze away from cleaning areas apply adequate coats of glaze - four is often recommended allow each coat to dry How Colors Develop Many ceramic glazes need to be fired in an oxidizing (air) atmosphere for best results. Red, orange and yellow glazes in particular are very oxygen sensitive. This means they require sufficient air during the firing to bring out the colors to their fullest and to prevent surface/ finish defects. Firing reds requires us to control the firing rate and properly vent the kiln. Controlling the Firing Rate Nearly all ceramics fire better when fired slowly below red heat. Slow firings have the advantage of allowing the necessary physical and chemical changes to occur in the ware. Slower firings also permit time for sufficient air to enter the kiln and displace the carbon monoxide. This is true for both bisque and glaze firings. Firing rate can be controlled using the settings on an automatic kiln, programming an electronic controller or by adjusting the switching. Control or slowing of the firing rate is most important in the early stages of the firing when most of the reactions are occurring and when air is needed to burn out the organics in ceramic materials. Near vitrification (the end of the firing) a faster rate is desirable and can usually be applied. Venting for Proper Air It is most important that enough air gets into the kiln in the early stages of firing. This is when the organic materials are burning out of the ware and air reacts with carbon to form carbon monoxide. Kilns can be vented manually or with an automatic venting system. Manual Venting Manual venting lets the fumes out of the kiln, but is only somewhat successful at letting air into the kiln. For manual venting, the top lid should be propped and the peephole plugs out for at least the first hour and a half. Slower firings require additional time. When the kiln reaches red heat, the lid can be closed and peephole plugs replaced. Leaving the peephole plugs out for the whole firing is not recommended since it can cause cold spots in the kiln. Manual venting works better with a smaller load. Also, using split level shelves allows air circulation and helps ventilation. Manual venting is recommended whenever a downdraft vent is not available. When venting manually, it may be desirable to locate red glazes on the top shelf to assure sufficient air. Automatic Downdraft Venting A downdraft automatic venting system like the Orton KilnVent efficiently brings the proper amount of air into the kiln and removes the fumes for exhausting. The kiln lid and peepholes remain closed the entire firing. Using the Orton Vent, test have shown reds can even be fired with other colors with good results. Firing to Proper Cone Number Using witness cones on the kiln shelf to verify results is important to good results. Many problems occur when red glazes are not fired to the proper cone number. Blistering can occur if underfired and loss of color if overfired. Glaze on underfired bisque may craze. Firing lead free glazes to the proper cone number is especially important. Firing reds can be a challenge, but by following good preparation, application, firing and venting practices, and by firing to the proper cone number, most problems can be eliminated.

Glaze Defects and Corrections

 

1.BISQUE AND GLAZE FIRINGS

On manual controlled kilns the total time to reach both bisque and

glaze firings should be at least ten hours.

• Always load the glaze kiln as densely packed as possible with pots.

If there is not enough ceramic ware to fill the kiln place posts and

shelves in the kiln to obtain a slow even firing.

• In the bisque firing the ware can be stacked together with pieces

touching. In the glaze firing the ware should be physically separated.

 

 

COMMON GLAZE DEFECTS

Listed are the most common glaze defects. Often if a defect can be

correctly identified a suitable correction can be enacted with a successful

result. One of the first stages in determining the cause of any

cracking problem is to find out when the cracks started in the ceramics

process. By carefully examining the type of crack (sharp or round

edge) it will indicate if it occurred before or after the glaze firing.

Determining when a crack started is a significant step in correcting a

defect in future ceramic pieces.

Round Edge Crack

A round edge crack has developed where the glaze rolls back from

the edge of the crack. Round edge cracks occur when the bisque is

already cracked and the glaze is applied over the crack. Some cracks

in the bisque are very small and go unseen however; once the glaze

is applied and the bisque is fired the crack increases in size.

Prevention:

• do not stack bisque in storage, do not drop bisque

• Inspect all bisque before glazing

Sharp Edge (hairline crack)

A sharp edge crack in the glaze fired piece indicates it occurred after

the glaze had “set” and became hard in the glaze firing process.

When this type of crack occurs it indicated a bisque and glaze not

fitting correctly.

 

2. GLAZE DEFECTS/CAUSE & CORRECTION

At some point there will be a defect when firing either clay or glazes

however, bad clay or glaze results can be kept to a minimum by

understanding ceramic materials. It is most important to diagnose

the problem quickly and apply a solution in order to fulfill orders and

insure good customer relations. The bisque products that you are

using in your studios are formed and fired to the highest standards.

We recommend examining all shipments of bisque products and

report any discolorations or irregularities in the bisque surface. Keep

all bisque clean and dust free before glazing. Often a few simple

guidelines will greatly improve the look and finish of the ware.

 

GLAZING AND FIRING RECOMMENDATIONS

Many glazing and firing problems can be avoided by following a few simple guidelines.

 

• The bisque should be clean and dust free to insure a compatible

glaze fit.

• The underglaze and glaze application thickness should not exceed

the width of a dime.

• To prevent damage to kiln shelves use a light application of the

recommended kiln wash.

• When glazing the bottom of ceramic pieces stilts (star shaped ceramic

supports which are used under the ceramic form) can be used to keep

the ware from direct contact with the kiln shelve. The ware can also

remain unglazed on the bottom and placed directly on the kiln shelve

without the use of stilts.

• All clay objects must be thoroughly dry before the bisque firing.

Clay contains mechanical and chemical water, which must be slowly

released during the first stages of the firing. On computer controlled

kilns always use the SLOW or MEDIUM  firing speed mode.

Correction:

• Try firing the glaze kiln one cone higher or lower to promote a

compatible glaze fit.

• Glaze the inside and outside of the bisque to equalize glaze fit.

• Use another glaze manufactures product, which might fit the

bisque compatibly.

• Often a thinner glaze application will stop a sharp edge crack.

• Slow cool the glaze kiln.

 

3. CRAZING

 

Crazing is one of the most common glaze defects luckily; there are

several corrections, which can eliminate crazing.

Glaze Crazing

Crazing is a fine network of lines in the fired glaze surface. Glaze crazing

occurs when the glaze is under too much tension as it cools in the

kiln. The bisque and glaze are not fitting upon cooling.

Corrections:

• A longer glaze firing often corrects glaze crazing. Try firing the kiln to

cone 06 in at least 14 hours.

• Firing one or two cones higher also improves glaze fit. Try firing the

kiln to cone 04.

• A thinner glaze application can stop crazing.

• Using another manufacturers glaze can stop crazing.

Glaze Shivering

Shivering looks as if “paint chips” are flaking off of the fire glaze.

Shivering is the opposite from crazing. In shivering the glaze is under

too much compression when cooling on the pot. Again the bisque

and glaze are not fitting upon cooling.

Corrections:

• Using another manufacturers glaze can stop shivering.

• A thinner glaze application can sometime stop shivering.

• Firing the glaze kiln one or two cones higher or lower can stop

shivering.

Glaze Crawling

When a glaze crawls it rolls back on itself sometimes exposing a bare

spot in the underlying bisque. Crawling can occur when one glaze

overlaps another or a glaze application is too thick. If the raw unfired

glaze is cracked or extremely dusty it can crawl in the glaze firing.

Corrections:

• Apply a thinner application of glaze (most glaze applications can be

applied thinner than the width of a dime).

• When overlapping glazes a thin application for both glazes is

required.

• Dusty or dirty bisque can cause the glaze to crawl. Always cover

bisque pieces until glazing.

• Some glaze are more likely to crawl and simply choosing another

glaze is the best solution.

 

4. Glaze Blistering

 

There are several possible causes of glaze blistering. Some glazes

when over fired can run and blister. A blister defect has a sharp crater

edge. Another cause of glaze blistering occurs when the glaze kiln is

fired at too fast a rate of heat increase. The immature glaze can blister.

If a glaze has been fast fired it can often be placed back into a

glaze kiln to remove the blisters. Re-firing cannot remove blisters

from over firing.

Corrections:

• If the blister is caused by a fast glaze firing re firing the piece will heal

the blisters.

• A longer time to the recommended glaze firing temperature is

required with new and re fired glazes.

• The kiln should be fully loaded with pots, if there are not enough pots

to fill the kiln place posts and kiln shelves to achieve a dense stack.

• If the blister is caused by over firing new glazes should be fired one or

two cones lower.

Fast Glaze Firing

Some glazes when fired too fast will have bubbles suspended inside

the glaze. Often they block out a color underneath the glaze. Many

glazes go through a stage where they bubble during the firing.

Bubbles in the glaze are more apparent in clear, transparent glazes.

Glazes need increased time to reach their maturing temperature and

become clear and smooth.

Corrections:

• Try a slow glaze firing to the glaze maturing temperature.

• Often firing one cone higher will remove bubbles from a glaze.

• Re firing a glaze will give it more “heat work” and remove bubbles.

• Stacking the glaze kiln tightly will increase the thermal mass during

the firing.

• A thinner glaze application will stop bubble held within the fired

glaze.

Glaze Run/Drip

Glazes can sometimes run or drip under certain application, and firing

conditions. The drip can cause the glaze and underlying bisque to

stick to the kiln shelf. If the bisque cannot be removed from the shelf

try using gentle pressure with a chisel placed at an oblique angle

between the stuck glaze and kiln shelf. In some situations where one

glaze is applied over a second glaze it can cause both glazes to run.

Always test when applying one glaze over another.

Corrections:

• Use a thinner glaze application (most glazes should be applied slightly

thinner than the thickness of a dime).

• Fire the kiln one or two cones lower.

 

5. Crystal Glaze

 

Certain glazes can grow crystals upon cooling (devitrification) in the

glaze kiln. The crystals frequently look like white specks in the glaze,

which can cause the glaze to change color and opacity in random

areas.

Corrections:

• Cool the kiln at a faster rate.

• Stack the kiln with more space between pieces.

• If a glaze grows crystals in a particular part of the kiln use other glazes

in that section of the kiln.

• Often a similar glaze effect can be achieved by using another glaze

that does not grow crystals.

 

SUMMARY

While no body of information and advice can cover every possible

glaze defect a majority of problems start with the glaze application

being too thin or too thick. Fast glaze firings with kilns not packed

with pots, shelves, or posts are the next statistically proven areas for

glaze defects which include, pinholes, blisters, and changes in glaze

color/texture. The simplest advice, which is sometimes hard to follow,

is to test every glaze before committing a great deal of time and

effort into a project. An accurate procedure for glaze testing should

entail using the glaze on at least three vertical pieces placing a test

on the bottom, middle and top kiln shelves. It is important to protect

the kiln shelf under each test piece. It can cause both glazes to run.

Always test when applying one glaze over another.

Question: What is hotter?  Cone 6 or cone 06?

Answer: cone 6 is much hotter. Cone 6 is considered high fire and is well over 2000 degrees. Cone 06 is low fire and is in the 1800's. Cones go in this order.

Most popular sizes from cooler to hotter:                    &n