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Glaze Blisters

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?

Is the glaze recipe the problem?

• 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).
• 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 (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)?

Is the system is intolerant of gases?

• 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.
• Soak the kiln at maturing temperature to give the glaze a chance to heal itself.
• 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.

In gas kilns:

• 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.
• Use a body that generates less gasses 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).
• 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 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.

Is the problem glaze application?

• 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, maybe you could switch to a different method.

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. This is actually a job for INSIGHT.

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