While there is no slam-dunk method for fixing or adjusting glazes, knowing how to detect the problem and approach fixing it, can make all the difference in the solution.
Detecting a Problem
Underfired glazes are usually matte and dry and can feel rough. Some gloss glazes seem like they were properly fired until you either look closely or use the object. For example, if you were to drink tea from a mug glazed with an underfired gloss, you might notice signs of crazing due to poor bonding, which become accentuated by the tea or coffee stains.
Glazes are complex and have many different melting points. The temperature at which a glaze melts and is considered mature depends on the balance of the three main glaze components in your glaze mixture. Silica forms glass. A flux makes otherwise very high-temperature melting silica (3110°F (1710°C)), melt at lower, more attainable temperatures such as 2232°F (1222°C) (cone 6). Alumina is added as a stabilizer to essentially keep the melted silica adhered to the ceramic surface. Making a glaze that’s compatible with your clay and your firing temperature is all about balancing these materials.
If you open your kiln and your glaze looks underfired (for example, glossy glazes appear matte or the surface has an orange-peel texture), you have several factors to consider. To start troubleshooting, work from the easiest possible solution to the more complicated. This way you can systematically eliminate what caused the underfiring by starting with the simplest faults first, before speculating about more serious and time-consuming problems. After addressing the easy problems, if you determine that the glaze is the problem, you can alter the recipe.
Defining the Terms
Glaze Slurry: The mix of glaze materials and water. Controlling slurry thickness is important to get a glaze to melt correctly.
Heat Work: The integrated effects of both temperature and time; heat saturation.
Specific Gravity (or Relative Density): The measure of how much more or less dense a specific liquid is than water.
Unity Molecular Formula: Also known as a Seger Formula. Glaze recipe format based on calculating the number of molecules of flux in a glaze to a total of 1.0 (which means they are in unity). The formula shows the ratio between fluxes, but perhaps more importantly, it shows the ratio of combined fluxes to the silica and alumina.
Viscosity: The resistance to flow; anti-flow. Low viscosity flows a lot. High viscosity flows only a little or not at all.
Troubleshooting: Easy Fixes
If the underfired glaze is one you often use and it normally melts at your firing temperature, then you can look for these potential technical problems as possible causes for the under-matured glaze result.
Did the kiln reach the right temperature? When you open your kiln, and all of the glazes are underfired, then you can go ahead and check whether your kiln fired to temperature. The truest way to affirm how high your kiln fired is by placing cone packs in your kiln. Usually they are placed at different levels in the kiln, as temperatures can differ throughout a large and/or older electric kiln—typically, colder at the bottom and hotter at the top. Newer electric kilns are shamelessly even; so one cone pack on the middle kiln shelf can be sufficient. With a computer-controlled kiln, there will be a message on the digital display that the kiln fired as you programmed it. It will also give you a message if an error occurred during the firing. Hopefully, it will be clear to you what went wrong (maybe a power outage, or a circuit overload, etc.) and you can easily restart your kiln without a visit from an electrician.
Check your firing program: Did you set your kiln to fire to the right temperature? At this point in my career, I need reading glasses to ensure that I have keyed in the right temperatures, ramps, and holds. Glasses or not, small distractions can also lead to program errors. Check your program twice.
Troubleshooting: Intermediate Fixes
Placement in kiln: If you open your kiln and there are fired pieces with a good melt and others with the same glaze that are underfired, then we must look at where the underfired pieces are located in the kiln. This is an equipment-related issue again. Are they all near the same element or on the bottom of the kiln? If so, then I would suggest having a kiln technician or electrician measure the heating elements. Maybe you just need new elements or, because the kiln is big and old, you need to increase your top temperature or increase your holding time at your normal top temperature to get more heat over time, so that the glazes have time to melt and smooth out. If the placement of the pieces seem to be irrelevant, for example if you have well-fired and underfired pieces on the same shelf, then we increase the difficulty of isolating the problem from looking at equipment to your bisque firing and glazing procedures.
Bisque firing too low: Underfired glazes can be a result of bisque ware that is underfired. That would mean that the bisqued surface is too porous and when you apply the glaze, the surface absorbs too much of it. The result could be underfired glaze, because there is too much glaze on the pot.1
Try increasing the top temperature of your bisque firing, making your bisque ware less porous. This may be enough to help the otherwise good balance of materials in your glaze melt as desired.
Troubleshooting: Viscosity and Density
How is your glaze slurry? Do you routinely mix it and/or sieve it every time you glaze or do you leave it in a bucket for long intervals and mix it up just enough to get it fluid and suspended enough to dip or pour it onto your piece? If so, the materials may not be evenly dispersed throughout the slurry.
Is the glaze in the bucket too thick or too thin? Either of these can result in underfired glazes. Glaze slurry that is too thick can cause underfiring because there is simply too much glaze on the surface, and glaze that is too thin can cause the same problem, simply because there is not enough glaze to melt smoothly (1). Be consistent. Each time you use a glaze, mix it well, making sure that any materials that have settled are mixed up into the slurry. If you do not mix the whole bucket, you will likely get a glaze that is unbalanced because key ingredients—or valuable amounts of them—are still sitting at the bottom of the glaze bucket. I like to sieve my glazes if I have not used them for a while (more than a month).
Measure the specific gravity using a hydrometer if desired. Otherwise do an educated dip test (for glazes you know well). Dip a test tile or your dry finger in the slurry and bend your finger. You can determine how thick or thin it is by how much or little it coats your knuckle. Each time you dip or pour your applied glaze, give it a stir first.
Remember the thickness of your glaze layer can also depend on the porosity of your bisque ware. Having these two variables in sync can eliminate an underfired glaze (2).2
If it is a new glaze that is underfired, make tests using a thin slurry, an average one, and a thicker one. Measure the specific gravity of these tests using a hydrometer and keep a record of the test tiles, so you understand what you are looking at after they are fired. Then you will be able to duplicate the specific gravity of the slurry that gave you the best melt.
Troubleshooting: Complex Solutions
If none of the above troubleshooting solves your underfired glaze problem, then look at the glaze formula for a possible solution.
Glazes are complicated: Underfired gloss glazes often show crazing because they have not bonded with the clay (3). They are sitting on the surface of the clay and are shrinking more than the clay. If this is the case, then you need to explore both your glaze and your clay body to find the right fit. Silica is essential in making a glaze, as it is the glass former. Look at your silica levels. If your glaze is crazed, too little silica may be the cause.
How you approach adjusting your glaze depends on you. Calculating the unity formula will allow you to add and decrease materials until you reach the right ratios between silica, flux, and alumina. Glaze calculation programs make it possible to do this easily, as you can add and adjust raw materials to make many different glazes and see and compare their analyses. Another approach would be to make lots of glaze tests. By testing glazes and actually seeing the results, you will learn quickly how materials react to each other in your glaze. You can use both methods: adjust your underfired glazes with glaze-calculation software, and test all the glazes you think will work or find interesting. You will have hands-on results and see the glazes that work.
Matte glazes are softer than gloss glazes and can appear underfired. Matte glazes are matte due to the presence of crystals under their surfaces and also because of the balance of the first three important glaze materials; silica, flux, and alumina. In reality, many matte glazes are underfired. Matte glazes that are underfired are most likely low in silica (in relation to the other two materials.) One way to determine if the glaze is underfired is to run a knife across the surface. Underfired mattes can be easily scratched with cutlery (4).
What if your glaze does not melt at your target temperature? If all of the ideas above are eliminated as the cause for the underfired glaze, consider the following:
Is this recipe intended for the cone range you fire to? Do you want to use this particular glaze recipe so much that you will fire an entire kiln load higher than usual to get it to melt, or fire separately in a less than full kiln to get the results you want?
Because I think the ultimate way of learning glaze chemistry and thus achieving rewarding results from your kiln is to know your raw materials characteristics, I would attempt to adjust the recipe.
Getting an underfired glaze to melt at cone 6 oxidation, for example, usually involves adjusting the fluxes. By adjusting the fluxes, I mean you have to find out which flux to use through testing. Remember that you may need to adjust the silica as well once you get the right flux. Coloring oxides can enhance the melt as much as opacifiers can inhibit it. Finding the right balance will yield a gloss with a smooth surface.
There is no slam-dunk method for making or adjusting glazes. The difference between labeling a result as an effect or a fault depends on what your intentions are for the glaze (5–6).
Making lots of interesting tests, keeping good records, and understanding your materials and how they behave in the glaze balance form the hands-on way to learn how to adjust and lower the firing temperature of your underfired glaze. By lowering the maturing temperature, you may also slightly or completely alter the look of the surface. You may totally nail it. If it is not the glaze you are looking for, it is not a loss. You are one test closer to understanding how your materials work together and are learning what to explore in your next set of tests to fix the underfired glaze.
1 Frank and Janet Hamer The Potter’s Dictionary of Materials and Techniques, Underfired, pp. 347.
2 Frank and Janet Hamer, The Potter’s Dictionary of Materials and Techniques, Slop, pp 316.