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What Determines a Glaze's Firing Temperature?

Section: Glazes, Subsection: General


By understanding how glazes melt and materials and chemistry interplay to determine behavior and temperature of melting and testing degree of melt you control the melting temperature of your glazes.

Article Text

Just as many people are not what they claim to be, glazes often bill themselves as one thing yet prove to be quite another. A glaze recipe might claim to be for cone 6, for example. However, that label could well be strictly a matter of opinion! The truth is, glazes do not "melt" at one particular temperature, rather they 'soften' over a wide range. The glaze mentioned above may begin to melt at cone 02; but someone has chosen cone 6 to "freeze in" the developing melt for reasons specific to their circumstances and tastes. The rate of rise, total firing time, and rate of cool are all critical factors specific to individual situations. Thus, the main thing determining a glazes firing temperature is you. In fact, you are also the potential controller of that softening range (I will refer to temperature according to Orton cone numbers).

There are factors that make judging the firing range of a glaze much more complicated than it might at first seem. A crystalline material has a specific melting temperature at which the regular lattice structure is broken. A glass, on the other hand, has a much more random molecular structure (called amorphous). Thus, the bonds holding it together vary in their strength to resist increasing temperature. The result is that a glass melts gradually over a sometimes wide temperature range. This is called "softening". Glaze recipes are made from powdered materials that are amorphous (e.g. frit), crystalline (ailica), or both (e.g. feldspar). Each behaves very differently as temperature increases. A recipe that contains all of them will thus have a very complex melting process. An Orton cone itself is a fine example (it softens and bends many cones before it fully melts).

Individual materials in a recipe, typically, have a very wide diversity of particle sizes. In spite of what textbooks say about the development of eutectics, these theoretical processes occur predictably only with extremely fine and well-mixed ceramic powders. The materials having finer sizes will melt much more quickly. For example, visualize melting a large chunk of wax. Now grate it into small flakes and melt them. I think you see what I mean. Likewise, finer materials in the glaze will begin to melt long before others. As this occurs, they react with the others whose melting, in turn, accelerates. It is possible to stop a firing during this process and solidify whatever has occurred. Is such a glaze "mature?" To answer, consider that the rate of increase in temperature can significantly influence melting activity. Given more time, many more interparticle reactions will take place. Given less time and higher temperature, those components of the glaze having a lower melting temperature will become much more fluid and react less with other particles. In an extreme case, the former will produce a well-developed glaze; the latter a matrix of unmelted particles barely held together by a glassy glue. Of course, you want to use your fluxes as molecular building blocks in a glaze, not as interparticle glue. So for a glaze to be mature, one would expect that at least enough temperature and time has been applied to melt most of the mass. At a minimum, it should be a very stiff melt. If a glaze lacks lustre and hardness, then it is possible it is being frozen at this early stage. Then again, maybe that's what you want; so for you the glaze is mature.

The greater melt fluidity provided by more time and temperature in the kiln affords more molecular mobility. Given sufficient freedom to move, the molecules will arrange themselves in an increasingly preferred matrix and cooling will freeze this as a solid. Depending on the above and other factors, the temperature range over which this occurs can be surprisingly wide.

So, where should it be halted? The simple answer is: At the body's optimal maturing point. As a glaze becomes more and more fluid, it begins to react with the body to form an interface of layers of intermediate compositions. The better this interface develops, the stronger and more functional the ware will be. So, as you can see, an ideal clay-glaze combination is one where both reach optimal temperature at the same time. Remember, also, that optimal body temperature is not always the temperature of highest fired strength. It is more likely to be a temperature of compromise between fired strength and resistance to warp in the kiln; a 'window' within which a particular visual effect occurs or simply the lowest practical temperature at which a target strength is achieved. So, what is ideal to produce a mature glaze? Only you can say but it appears it is important to have some control to optimize a glaze to the body's ideal.

Since the body and glaze themselves produce gases of decomposition during development, the fluid glaze has to pass these. Early in the fluid cycle, these bubbles are abundant and as thermal soaking or temperature rise continue, they have opportunity to work their way out and break at the surface. At what point is the glaze mature? Only you can say.

Once a glaze has melted completely and established free movement of oxide molecules, another process is under way. The fluid melt is stiff and viscous at first, but as temperature increases, it thins and eventually will either run off the ware, boil, volatilize, soak into the body, or react strenuously with it. Interestingly, if you slow-cool a glaze fired well into this stage, it can produce very matte surfaces. This is because a network of very fine crystals develops at the surface. The slower the cool and the higher the temperature, the better the development. One other factor is worth noting. Some fluxes, like MgO, act as refractories until a specific temperature where they suddenly flow and become active. Glazes frozen in this range have a distinctive mottled effect resulting from the uncombined MgO actively flowing and 'feathering' itself throughout the viscous melt.

Thus, there is no simple answer to what temperature a particular glaze fires best at. There are so many materials, process, and finished product considerations that it is impossible to make any rules, except one: Flow and tile test your glazes at many temperatures so that you know what you have (see the links for and flow tester). I have another suggestion also: Don't use too many recipes. Strive toward using a few, or maybe variations on just one. In this way, you will be able to study and know the glaze(s) you use and then be in a position to adjust and control them. It is better to be on friendly terms with one than enemies with twenty.

There is ample economic and environmental incentive in both hobby and industrial circles to fire kilns to the lowest temperature possible. However, there is understandable resistance to changing existing formulations that have been working. Body recipes might also need adjustment, since they could lose strength and density in the process. On the other hand, changes in body materials may mean it is desirable to fire a little lower. Admittedly, in some cases it is actually better to fire higher, since lower temperatures sometimes require more expensive and highly processed materials, many of which are environmentally hostile. They also require more testing and control, and lower temperatures are not as forgiving. In addition, some glazes depend on a melting or freezing behavior that is specific to a narrow range of temperatures (i.e. MgO).

Still, using proper preparation, it is frequently possible to achieve equal or even greater strength in body and glaze at a lower temperature. Sometimes just moving one cone lower can mean significantly less stress on the kiln and an appreciable reduction in fuel consumption.

Most people don't realize how easy it can be to adjust the average balanced glaze to a new temperature. Let's look at two formulation approaches to this problem. Since these depend heavily on your ability to do many quick recipe-to-formula calculations, INSIGHT is indispensable.

Before attempting to reformulate a glaze to a lower fire, consider if it is necessary. If the glaze to be adjusted is a glossy or matte base with opacifier or colorant added, then do you already have a similar base at the lower temperature? Will it work with the same additives as is or with minimal adjustment? If so, then use it. If not, then let us continue.

First, what is different about formulations for high and low-temperature glazes?

Here is an example of approximate oxide ranges for Orton cone 6 and 10 lead-free standard whiteware and pottery glazes (the figures to follow compare the numbers of oxide molecules of each assuming flux unity).

  Cone 6 Cone 10
CaO 0.2-0.5 0.4-0.7
ZnO 0.1-0.3 0-0.3
BaO 0.1-0.3 0-0.3
MgO 0-0.2 0- 0.4
KNaO 0.1-0.3 0.2-0.4
Al2O3 0.2-0.3 0.3-0.5
B2O3 0.3-0.6 0.1-0.3
SiO2 2-3 3-5

It is the amount of SiO2, B2O3 and Al2O3 whose proportions really determine the melting temperature of a glaze. Also fluxes are more diversified. Notice that the Al2O3 and SiO2 are about one third less for cone 6 than cone 10. Also B2O3 materials can melt as low at cone 06, so increasing it could be a real help to reduce a glaze's firing temperature.

The glaze that I want to adjust is a cone 10 reduction celadon, and I wish to bring it down to cone 6R. This is quite an ambitious undertaking. Although cone 6 and cone 10 may not seem far apart on paper, there is usually a vast difference between clays and glazes intended for each. To be honest, there are many recipes that cannot be converted without the introduction of more active fluxes that can change the visual character. Recipes that have abundant kaolin, ball clay, feldspar, and silica are often prime candidates for change. On the other hand, as already mentioned, don't convert a recipe if the same base type is already available at the lower temperature. In this case if you had a celadon-like stiff-melt clear that suspends micro-bubbles and reacts with iron to give green, then no conversion would be necessary.

Here is a detail calculation of the glaze I want to change

DETAIL PRINT - Cone 10R Celadon
MATERIAL               PARTS  WEIGHT   CaO*   MgO*   K2O*  Na2O* Fe2O3*  Al2O3   SiO2
WEIGHT OF EACH OXIDE                   56.1   40.3   94.2   62.0  160.0  102.0   60.1
-------------------------------------------------------------------------------------  MATERIAL
Expan OF EACH OXIDE                    0.15   0.03   0.33   0.39   0.13   0.06   0.04   Cost/kg
-------------------------------------------------------------------------------------  -------
 CUSTER FELDSPAR....   25.50  617.10   0.00          0.03   0.01   0.00   0.04   0.29     0.00
 WHITING............   14.00  100.00   0.14                                               0.12
 KAOLIN.............   19.00  258.14                                      0.07   0.15     0.24
 SILICA.............   31.00   60.00                                             0.52     0.19
 IRON OXIDE RED.....    4.00  160.00                               0.03                   2.90
 DOLOMITE...........    8.50  184.00   0.05   0.05                                        0.00
-------------------------------------------------------------------------------------  -------
TOTAL                 102.00           0.19   0.05   0.03   0.01   0.03   0.12   0.96     0.23
UNITY FORMULA                          0.63   0.15   0.09   0.04   0.09   0.39   3.20
PER CENT BY WEIGHT                    11.79   2.09   2.88   0.86   4.56  13.33  64.49

Cost/kg  0.23
  Si:Al  8.21
 SiB:Al  8.21
  Expan  6.75

Notice the Al2O3 is in the middle of the normal range for cone 10 glazes and the SiO2 is at the low end of its range (the 'Unity Formula' line). To adjust this recipe to cone 6, the strategy will be simple: put the Al2O3 in the middle of the cone 6 range and the SiO2 at the low end. I will be retaining the SiO2 : Al2O3 ratio at around 8.0 and won't be touching the proportions of any other oxides, so the appearance of the glaze should be retained.

To reduce Al2O3 and SiO2 , first reduce materials contributing them. From the above printout, notice that kaolin and silica contribute both and contain no other oxides. Had this recipe lacked kaolin, I would have had to reduce the feldspar to cut Al2O3 and compensated for the loss of other oxides it contributed.

Following is a detailed calculation after the reductions were made

DETAIL PRINT - Cone 6R Celadon
MATERIAL               PARTS  WEIGHT   CaO*   MgO*   K2O*  Na2O* Fe2O3*  Al2O3   SiO2
WEIGHT OF EACH OXIDE                   56.1   40.3   94.2   62.0  160.0  102.0   60.1
-------------------------------------------------------------------------------------  MATERIAL
Expan OF EACH OXIDE                    0.15   0.03   0.33   0.39   0.13   0.06   0.04   Cost/kg
-------------------------------------------------------------------------------------  -------
 CUSTER FELDSPAR....   33.00  617.10   0.00          0.04   0.02   0.00   0.06   0.38     0.00
 WHITING............   18.00  100.00   0.18                                               0.12
 KAOLIN.............   11.50  258.14                                      0.04   0.09     0.24
 SILICA.............   22.50   60.00                                             0.38     0.19
 IRON OXIDE RED.....    4.00  160.00                               0.03                   2.90
 DOLOMITE...........   11.00  184.00   0.06   0.06                                        0.00
-------------------------------------------------------------------------------------  -------
TOTAL                 100.00           0.24   0.06   0.04   0.02   0.03   0.10   0.84     0.21
UNITY FORMULA                          0.64   0.16   0.09   0.04   0.07   0.26   2.23
PER CENT BY WEIGHT                    15.88   2.82   3.90   1.17   4.79  11.98  59.46

Cost/kg  0.21  
Si:Al  8.42 
SiB:Al  8.42  
Expan  7.60

When doing this type of adjustment, keep a few things in mind. Kaolin was used in the original recipe instead of ground alumina to source Al2O3 and for good reason. Not only is it inexpensive but it acts as a suspender to keep particulates from settling. Reducing it to accomplish a reduction in Al2O3 is fine but to retain reasonable suspension sometimes you may have to add bentonite or switch to the more effective ball clay to supply the reduced Al2O3 quota. Also, if the recipe total changes, remember to maintain the same percentage of iron oxide (or other colorants, opacifiers, etc.) in the recipe.

Notice the calculated expansion has increased because of a reduction in the two oxides, which make the greatest contribution to keeping it low. This means there is a chance the glaze may tend to craze. Since middle temperature glazes have less SiO2 and Al2O3 , crazing is more common anyway. If glaze fit proves to be a problem, we could probably increase the SiO2 without adverse effects, some higher-expansion CaO could be exchanged for some lower expansion MgO, or B2O3 could be introduced at the expense of some of the high-expansion Na2O and K2O.

So far, I have just done a calculation and hoped for the best. An auxiliary approach is to make a line blend.

To do this I will calculate the kaolin to silica mix, which has the same SiO2 :Al2O3 ratio as the original glaze will remove the two materials from the formula in this ratio.

Shown here is a partial report of the results of a calculation to determine the kaolin:silica mix to yield a SiO2 :Al2O3 ratio of 8:1.

KAOLIN...........     5.00 
SILICA............    7.00
Al2O3   .19    17.49% 
SiO2   1.55    82.51%

Rather than mixing and weighing out a test batch of each blend, there is a simpler way. Mix up a test using 10 less kaolin and 14 less silica (the proportion just determined).

Adjust the recipe so that you retain the 4% iron; it should work out like this.

POTASH FELDSPAR 25.50 25.5 34.0
WHITING 14.00 14.0 18.5
KAOLIN 19.00 9.0 12.0
SILICA 31.00 17.0 22.5
IRON OXIDE 4.00 4.0 4.0
DOLOMITE 8.50 8.5 11.0
  ------ ----- ------
  102.00 78.0 102.0

Now, line blend this 75:25, 50:50, and 25:75 with the original. There is an easy way to do such a blend:

This method is very quick and line blends of 10 intervals are almost as simple to do as those having 4.

We have done only one glaze here but the technique is quite simple. This method assumes that the original glaze is not an unbalanced or critical eutectic mixture. Certainly, there are other ways to optimize melt temperature in a glaze. Sometimes you can do it by moving to a nearby eutectic mixture (with the help of the dreaded phase diagram). Also, the miracle oxide of low-temperature glazes, B2O3 , is always available to move a glaze down while maintaining its expansion. Small amounts of powerful fluxes like lithium or zinc can sometimes help. I leave it to you to explore some of these avenues. But for heaven's sake, don't just blindly throw in a frit.

One final thing: You need to evaluate the new lower melting glaze. The best approach is to use a flow tester to compare melt flow at the new temperature with flow of the original at the old.

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By Tony Hansen

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