Glaze Blisters

Questions and suggestions to help you reason out the real cause of ceramic glaze blistering and bubbling problems and work out a solution

Details

Blisters are evident on the fired glaze surface as a 'moonscape' of craters, some with sharp edges and others rounded. These craters are the remnants of bubbles that have burst during final approach to temperature or early stages of cooling. In some cases, there will be some unburst bubbles with a fragile 'dome' than can be broken. Blisters can vary in size and tend to be larger where the glaze is thicker. This problem is so serious that entire production lines can shut down when it hits. Generally potters will encounter this problem much more than industrial producers (the latter use more balanced glazes and cleaner clay bodies).

Is the glaze fluid enough?

Often glazes appear like the melt should have plenty of mobility to heal but this can be deceptive. A melt flow testing regimen is the only way to know for sure about how much your glaze flows and if flow is changing over time. Melt flow testers have a reservoir at the top of a steep incline, the glaze runs down a calibrated runway. Generally, a fluid glaze will heal blisters much better, but only if it has time and if they have broken.

Is the glaze too fluid? Is the surface tension too high?

I am about to tell you something that could fix a blistering problem that has nagged you for years! It seems logical that a glaze not healing its blisters does not have enough melt fluidity. But very often, the opposite is the actual truth. How is it possible that a highly fluid melt can form blisters that do not heal? Bubbles can only form in a high-fluidity high-surface tension melt (analogous to the way bubbles form with soapy water). Reactive glazes are often of this type, those having high percentages of boron (e.g. 0.7 or more molar in a cone 6 glaze) so they melt well but at the same time having high surface tension. At the same time, such glazes often contain colorants and other materials, having high LOIs (producing a lot of gases of decomposition that become bubbles in the melt). Such glazes that form bubbles can be soaked for hours at top temperature and the bubbles may not break. If the temperature is dropping rapidly they may not break at all, and if they do at some point, what remains of the melt may not have time heal itself. This can happen even with fine porcelain bodies. Here is the key: Melt-fluid glazes need to avoid high surface tension. If that is not possible they need to be cooled to the point where the increasing viscosity of the melt is enough to overcome the surface tension that enables the bubbles to form and hold (of course that temperature is different for each glaze, it is normally found by decreasing the drop-and-hold temperature over a series of firings). This breaks the bubbles and provides enough time for the remaining melt fluidity to level out the surface. Each glaze has a different point at which this happens and ideally, cooling the kiln to just below that temperature and soaking there is the best solution. Failing that knowledge you can experiment with slow cooling the kiln through various temperature windows to discover the best results. Be aware that this temperature could be lower than you think, be prepared for it to be 200 degrees F or more below the firing temperature. Of course, if you are able to adjust the chemistry to favor fluxes that have a lower surface tension that will also help a lot. There is a linked photo below that shows how you can recognize high surface tension on a flow tester.

Are excessive gases generated during glaze fire?

Significant amounts of gases can be generated within the glaze itself due to the decomposition of some materials after melting has started (i.e. dolomite, whiting, manganese dioxide, clays, carbonate colorants, etc). Substitute these materials for others that melt cleanly. For example, use frits, supply CaO from wollastonite instead of whiting or dolomite, use cleaner clay materials, or use stains instead of metallic carbonates. If you are using organic additives be aware that some of these can generate considerable gases during decomposition; do tests without them, use an inorganic substitute or find way to disperse them better into the slurry.

You might be under estimating the amount of gases that are coming out. Are you holding the top temperature long enough? Perhaps a much longer than expected soak might be necessary (on very thick tile or sculptural pieces, for example, 24 hours might be needed). Could you do a test on a small piece to confirm this? It might also work to adjust the firing schedule to soak, decrease the temperature a little (so the glaze is still fluid enough to level) and soak it there.

Is the glaze recipe or chemistry the problem?

The approaches to dealing with glaze chemistry issues differ in fast fire (e.g. tiles) and slow fire (studio pottery). In slow fire we want glazes that are mobile and can heal imperfections over a long soaking period. In fast fire we want glazes that remain unmelted until after 950C (gases from decomposition can occur up until this temperature) and then melt quickly after this.

If you are firing fast then you need to use a fast-fire glaze formulation so the glaze does not begin to melt until after body gassing is complete (the whole modern white-ware and tile industries are built on this principle). In fast fire, matte glazes automatically have this property because the formulations to make a crystalline matte and a late melting glaze are the largely same. Glossy glazes, however require extra attention.
Reduce zircon or alumina in the glaze melt to give it better flow properties. Or source them from a frit rather than raw materials.
Reformulate the glaze to have more fluidity to heal imperfections (i.e. more flux or a lower alumina:silica ratio).
Strontium carbonate can help smooth viscous zirconium glazes, small amounts of ZnO and Li₂O can do miracles for glaze flow.
Adjust the glaze so that it has a lower surface tension so that bubbles break more easily during soaking.
Boron can induce blistering, especially if its amount is quite high (check limit/target formulas for guidance). The reasons for this phenomenon are not because of gassing (this is demonstrated by the fact that high boron glazes often blister worse on a second firing). Boron is a glass like silica and it wants to form its own glass structures. High boron can thus cause phase separation (areas of discontinuous glass chemistry in the fired glaze, e.g. globules of a sodium borate glass in a calcium silicate glass matrix). Considering the important function of alumina in glass structure, the lack thereof would be an aggravating factor in the separation. Phase boundary phenomenon and the differences in surface tension and melt fluidity of the phases could breed blisters. This process likely continues in a second firing (this accounts for blistering getting worse). Ferro Frit 3134, for example, has no alumina, lots of boron and plenty of CaO/Na₂O, glazes high in it are candidates for this phase separation.

Is the system is intolerant of gases?

Gas release from decomposing materials in the body can continue until 950C. Many glazes begin melting long before this.

In the single fire process (i.e. tile) gases have to bubble up through the glaze if it melts too early. The most important factors in producing flawless glaze results in single fire ware are a dense properly pugged or pressed clay matrix that is not too thick, the use of fast-fire glazes specially formulated to melt as late as possible, a firing curve that recognizes the need for a slower rate-of-rise at glaze finish temperatures, and a body made from clean materials and containing a minimum or organics.
Use a body of finer particle size so that gases are channeled to more surface sites of lower volume help, individual sites do not overwhelm the glazes ability to pass them.
Minimize techniques that roughen or remove fines from the leather hard or dry clay surface of bodies that contain coarser particles. If necessary apply a fine particled slip to leather hard or dry ware to filter internal body gases into finer bubbles during firing.
Apply the glaze in a thinner layer to minimize its ability to contain large bubbles.
Use clays not containing large gas generating particles (i.e. pyrites, sulphates)
Some fluid glazes (e.g. rutile-blues) tend to be quite sensitive to blistering. This seems out-of-place since the glaze is fluid and should be able to heal imperfections. But the problem is often that they melt early, seal the body surface, and percolate escaping gases. After all gases have been generated they still have enough surface tension to support populations of unborken bubbles containing the last of them. The fast temperature drop of the kiln shut off does not provide adequate time for them to heal or even break. Experiment with firing curves to learn where to slow cool-down and give them a chance to heal when the melt is stiffer (this might be much lower than you think, e.g. 1400C for a cone six version).

Is the glaze firing part of the problem?

Fire the glaze higher or adjust its formulation so that it melts better and more readily heals surface bubbles.
In a slow-firing setting, you may need to soak the kiln longer at maturing temperature to give the glaze a chance to heal itself. In a fast-fire you need to do the opposite, soak only long enough to melt the glaze but not long enough to allow bubbles to grow.
Fire the kiln slower during the approach to final temperature.
It is not easy to understand why very fluid glazes sometimes do not heal blisters well. Sometimes they are not as fluid as they appear, do flow testing to find out. Or if they are fluid it may be possible that they need to be cooled slower through the transformation process at which they begin to stiffen and solidify; this can be hundreds of degrees lower than the actual firing temperature (if you are not using a fast-fire type glaze).
Rather than trial and error firing tests to find a schedule that is sympathetic to your body-glaze combination have your body evaluated for TGA and DTA. Thermal Gravimetric Analysis provides information on body weight loss during the whole firing curve so it tells you when gases are being generated. Differential Thermal Analysis shows where in the firing curve the body behaves endothermically and exothermically. An expert can use information from these tests and others to tune a firing schedule perfect for your situation. In the USA The Orton Ceramic Foundation can do this type of evaluation.

Is it being fired in a gas kiln?

Avoid very heavy reduction followed by periods of oxidation.
It is best to start reduction one or two cones higher than the bisque temperature, this period in the glaze kiln can oxidize any remaining potential 'blister producing' volatiles that the bisque did not take care of.
Avoid flame impingement on the ware.
Make sure that stage one of the glaze fire is truly oxidizing to avoid buildup of internal carbon in the body.

Is the body the problem?

Does the bare fired clay have a glassy film? Soluble salts within the body can move out to the surface during drying. If these are high in fluxing oxides they act as a very reactive intermediate layer between glaze and body. This can amplify existing pinhole contributors or produce glaze surface irregularities that are akin to pinholing. Add barium carbonate to the body mix to precipitate the solubles within the body or substitute implicated materials in the body batch.
Is the ware being dried too slowly after glazing? In industry great care is taken to accelerate drying to minimize dissolution of compounds by the water from the glaze (and to minimize issues with glaze adherence that extended drying times can raise). Compounds that can cause blistering are thus trapped inside the body.
Use a body that generates less gases of decomposition. For example, the tile industry uses low lignite ball clays both to enable fast fire and to get better glaze surfaces. Clean ball clays (those with low carbon) can be surprisingly light in color, some resembling kaolins (i.e. Spinks Champion ball clay).
Does the body contain barium carbonate particles? Screen a sample on a 200 mesh sieve. Does it contain non-white particles or unground barium? Contaminated barium can cause severe pinholing.
Is the body too dense to enable the gases to freely escape during the period before the glaze melts
Blisters and pinholes can share the same causes. Check the article on pinholing for more information on body problems that can cause glaze defects.

Is the problem in the glaze mixing?

Most companies ball mill their glazes and for good reason. It is not uncommon to mill glazes up to 12 hours. Blisters and surface imperfections are often caused by impurity particles in the glaze layer itself, grinding these down as small as possible will minimize the ability of individual ones to cause problems. Don't assume your ball mill is working, some configurations will not grind a glaze fine enough no matter how long they run.
A variety of contaminants can find their way into glazes and certain materials are potent sources of bubbles (e.g. plaster). Silicon carbide is an example, if you are doing any cutting or grinding using an SiC abrasive particles could be finding their way into your process (you might consider using alumina based abrasives).

Is the problem glaze application?

Have you changed the way glaze is applied? Have you changed employees doing the glazing? Do you have a quality control mechanism to record aspects of the glaze lay down (e.g. thickness, density, microscopy, resistance to rub-off/dusting). A thicker glaze application is, of course, much more likely to blister.
Many companies target very high specific gravities in their glazes (i.e. 1.8) to achieve a dense laydown (your application method may limit this). This minimizes entrained air and thus imperfections. Certain application techniques produce a better laydown, others produce a fluffier layer (e.g. spraying). If this is your case, think about ways to densify the dry layer.
Do not put wet ware into the kiln, a variety of problems related to the nature of glaze laydown can result. It is surprising how high the temperature can get and yet steam still be present inside kilns without ventilation.

Are you bisque firing? Is it done right?

All clays release gases from burning of carbon material and decomposition of other compounds. Some clays release sulphur compounds also. If the glaze is melting during release of these gases, they must bubble up through it. If the melt is stiff, the kiln is ramped up too quickly, cooled too rapidly, or the glaze melts too early, it will not have opportunity to heal properly.

Make sure the bisque fire has good ventilation, has a clean oxidizing atmosphere, is long enough, and that ware is stacked to expose maximum surface to oxidation. Tightly packed electric kilns lacking a venting system require extremely slow and thorough firing (especially through the red heat to 900C range). The superior ventilation in gas kilns makes them best for bisque firing. However, it is important to realize that heavy ware fired in a large electric kiln cannot be fired evenly, no matter how long it is soaked. This is because the element side will always deliver more heat work than the shady side. To appreciate this fully, go outside on a hot day, step into the sun, then into the shade!
Bisque fire as hot as is practical (cone 04-02) and vindicate bisque temperature with standard cones. A hot bisque is necessary to burn out any sulfur that might be present. A hotter bisque means denser ware and it may be necessary to adjust glazes to be thixotropic so they will apply well to the less absorbent body. Although you may not be accustomed to glazes that will stick to less absorbent bodies, be assured that this is very feasible. One caution however: If you ware is burnished, it is not usually advisable to bisque above cone 08 or the burnish can be lost.
Bisque fire as slow as is practical. Slow fire through the period where the most gases are generated from the oxidation of organics in the body (usually from 700C to 950C). 50C per hour is considered 'slow' If you have an electronic kiln controller experiment using a fast firing curve slowed down at various temperature ranges. This will help you determine the range at which it is most critical to fire slower. Make sure that reduction does not occur during any phase of bisque or reduced iron (FeO) could play havoc with latter stages of the firing).
If you do not have humidity drying equipment, candle periodic kilns overnight before bisque firing the next day. This will assure that ware is completely dry and that firing can proceed quickly to past red heat, leaving more time for the carbon burnout phase.

Do blisters get worse even if you fire ware again?

This may be a vindication of suspicions that a highly fluid melt of high surface tension is supporting bubble formation and existence. On the second firing the melt will be even more fluid than the first. It is doubly important that you identify the on-the-way-down temperature at which the glaze melt is too viscous to support stretching a bubble but fluid enough to still level out. When you find it, soak the kiln there.