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.
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 (melt flow testers have a reservoir at the top of a steep incline and the glaze runs down a calibrated runway). Generally a more fluid glaze will heal blisters much better (see section below on blisters occurring even after refire).
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 pretty fluid), hold it and then cool quickly for the next few hundred degrees to solidify the glaze.
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 whiteware 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 at the surface.
• Does the recipe contain binders? When do these decompose to create gases (it might be higher than you think)?
• 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 agravating 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 make ideal 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 channelled to many more surface sites of lower volume and thus do not overwhelm the glaze if they have to bubble through it.
• 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 (i.e. rutile-blue) tend to be quite sensitive to thick application and fast firing and cooling and bubbling problems with them seem out-of-place. Experiment with firing curves to learn where heat-up or cool-down rates need to be slowed.
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 or down through transformation 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. 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 firing 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. Watch the kiln to make sure there is plenty of oxygen present at all times.
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 firing? In industry great care is taken to accelerate drying to minimize dissolution of compounds by the water from the glaze (and too minimize issues with glaze adherence that extended drying times can raise). These compounds, that can thus 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. The ball clays 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 free 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 thinking 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 the kiln.
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.
• 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 stage 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 often happens and it is not easy to understand since one would think that there can be no source of gases if the piece has already been glost fired. Regardless of the reason if a glaze is not healing its blisters on multiple firings then it is not fluid enough. One does not fully appreciate how stiff the average glaze melt is until you work with crystalline glazes that are so fluid a bowl must be placed under the ware to catch the runoff. However the fired surfaces of these glazes are incredibly glossy and perfect. If your glaze melted more it would run more, however you can counter this by putting it on thinner. The melt fluidity of a glaze is primarily affected by the amount of flux, so you need to increase it. However if the flux you choose has a higher thermal expansion be prepared for the glaze to craze.