'Pinholes' are small holes in the fired glaze surface penetrating down to the body below, often into a surface pore or opening. 'Pits' are smaller, they mar the surface but to not penetrate all the way down. Pinholes or pits are often no larger than the head of a pin. During firing bodies typically generate gases associated with the decomposition of organic materials and other minerals, escape of crystal water, etc. If ware is glazed these gases may need to bubble up through the glaze melt, depending on how early it begins to melt. The causes of pinholes can often be similar to those of blistering. Keep in mind also that larger pinholes may actually be crawling (see links to other articles). In the following I may confuse pinholing and pitting or may neglect to mention one or the other, I apologize for this.
When pinholes or pits occur there are often more than one contributing factor. Generally a true pinhole is a problem with the body that extends up into the glaze whereas a pit could be considered a problem with the glaze or the firing. Still most strategies to eliminate these involve attack on several fronts:
• Reducing burn-off by higher bisque or cleaner body (less lignite for example)
• Distributing body out-gassing by finer grinding
• Giving the gases more time to escape by slower firing or using a fast-fire glaze that melts later
• Giving the glaze time to heal by soaking or slower cooling
• Providing more kiln draft to oxidize and carry away products of decomposition coming from the body or glaze
• Making the glaze more fluid or altering its surface tension to enable it to better heal itself
• Selecting glaze materials that decompose to form less gases
• Being careful to apply a dense even lay down of glaze.
Hobby and small scale producers have the flexibility to do much longer firings and generally must do so for the lack of fast-fire equipment and materials. Industrial producers must find ways to fire quickly, often in an hour of less. Strangely, even though small scale producers fire much slower, they can have just as many problems with pitting and pinholing. Some are using prepared bodies and/or glazes and thus have less flexibility to change things. Keep this factor in mind as you read the material below, the world you are in will determine the validity of the comments being made.
If a pitting or pinholing problem has started to happen and it has not occurred before do not assume that there is some new problem. If reading this article makes it clear that there are some things that you have been overlooking, then the success you have had up until now might be accidental. This may be an opportunity to make your process better and more stable.
Is the body the problem?
Are large particles or gas producing materials present?
Do a sieve analysis of the body to determine if large particles are present. Weigh, fire to cone 04, and re-weigh a sample of the coarse particle material to see if it loses significant weight (due to decomposition and associated gas generation). If the particles are volatile (i.e. lignite, sulfur compounds) they will generate high volumes of gases at individual sites, possibly overwhelming the glaze's ability to heal itself there. The most practical solution is to either remove the implicated material from the body batch in favor of a finer particle grade (to distribute gas generation to more sites of less volume) or use a cleaner alternative (by cleaner I mean low-lignite and low-sulphur ball clays).
Are the particles melting vigorously?
Use a sieve to isolate some of the coarser particles and fire them to body temperature. Fire to see fi any of them are active melters. Examine pinholes under the microscope so see if a glassy pool exists at its base. If this is the case it is possible that a combination of vigorous melting activity and the resultant creation of a glass chemistry that resists pinhole healing could be occurring. In this case, the offending particles in the body must be eliminated or ground more finely.
Troublesome materials in the body?
If you can see 'white spots' and dimples on the glaze surface this suggests that pinholes and imperfections existed but have healed incompletely (these may also suggest that the glaze melt does not flow as well as its glossy surface might suggest, more flux or later melting might be needed). Even fine particled bodies can gas badly, especially if they contain materials like talc, dolomite, or whiting that release high volumes of gas. It is common for some talc to be used as a flux in middle fire bodies (e.g. 2-5%) and there is not really a practical alternative that is as effective and inexpensive. That means that the firing curve must take the decomposition of talc into account slowing down the firing when this occurs.
Are there soluble salts in the body?
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 can act as a 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 body too open?
What is the fired porosity of the body? Does it have an open porous structure resulting from many coarser particles or laminations and air pockets (e.g. from poor pugging or sand, grog, shale, unground clay in the batch)? If pores are networked in a body that produces alot of gases on firing then these gases escaping from within are channeled into the network and converge at high volume surface vents (gas volume may be too large for the glaze to heal). Use a finer particled body or perhaps a fine slip between glaze and body.
Is the body lacking maturity (not vitrifying)? For example, using a body intended for cone 10 used at cone 6 can actually impede the melt of the glaze since body silica and alumina can rob the glaze of some of its fluxes and therefore impede its ability to smooth out.
Is the body bisque surface rough or irregular?
If the body surface is rough (because it contains grog or sand, or the ware has been mechanically trimmed during leather hard stage opening imperfections in the surface), pinholes often occur as the glaze dries on the body. This is a poor lay-down and these raw pinholes may turn out as fired pinholes. In addition, a rough surface exposes pore networks inside the body to larger volume 'exit vents' that produce pinholes in glazes. You can prevent this by using a finer body, smoothing the body surface in the leather hard state after trimming, or by applying a fine-grained slip. You can also wash bisque ware (do not soak it) prior to glazing, this will tend to make the wet glaze application fill surface irregularities rather than compress air into the voids then have it blow back out as a raw pinhole a few seconds later.
Do you understand the gas evolution profile of the body?
There are many ways to study the characteristics of your body in this regard so that you can adjust your firing to slow down during the high gas evolution phases.
Is there a problem with the glaze recipe?
Do you use binders?
Glaze binders have been known to produce serious pinholing and pitting problems. Some decompose at higher temperatures than you might think. Switch to another binder that decomposes at a lower temperature, eliminate it if there is adequate clay to harden the dry glaze layer, or reformulate the glaze to melt later and more quickly using a fast-fire frit. Once again I ask, do you really need a binder, or could bentonite do the same job?
Are any glaze materials contributing to the problem?
Some glaze materials produce large volumes of gases as they decompose during firing (e.g. whiting, dolomite, talc, coloring carbonates). These materials can decompose as late at 1000C, if this is after the glaze has started to melt it means trouble. In serious cases the glaze may not just pit or pinhole, but it may blister, the problem can be reduced or eliminated by employing other sources of the needed oxides (i.e. wollastonite for CaO, frits for MgO). Calculation will be required to make the substitution (so that the formula stays the same).
Do you need a fast-fire glaze
In industry the chemistry of fast-fire glazes is well understood (e.g. they have zinc and lower boron, this produces a later melt). If you are fast firing and are not using a glaze formulated for fast fire then you will almost certainly be having glaze pitting and surface imperfections.
Is the glaze melt is too viscous?
If the glaze melt is too thick it will resist flow, impede the passage of gas bubbles, tending to trap them in its matrix. Most often a glaze melt is viscous because it is not melting enough. However even well melting glazes can have a chemistry that makes them resist flow (i.e. high alumina content) or they may contain a material like Zirconium that stiffens the melt because it does not go into solution. Using melt flow testers to gauge the melt mobility of your glaze is a good idea, it is very difficult to detect melt flow changes by simple inspection of a glaze layer. You might think that the melt is fluid enough, but only a melt flow test will say for sure.
Increasing flux content to produce a more fluid melt often works well to combat pinholes and pits. Sometimes very small additions of ZnO, SrO, or Li2O can have a dramatic effect on glaze flow. Sourcing fluxes from frit or using a finer particle size material will improve the melt flow also. Or, you could simply fire higher.
Likewise, a decrease in the Al2O3 content will make a glaze more fluid but could add unwanted gloss if you are using a matte. As already noted, if the glaze contains a melt stiffener like zircon, check to see if trading off some of it for tin oxide helps.
It is possible that the glaze may be melting too much and blisters associated with glaze boiling may contribute to surface imperfections, however this is more likely to cause blisters or be associated with soluble salts from the body boiling below the glaze. Try adding Al2O3 to the formulation and note an improvement to confirm this.
Is the glaze melt and sealing the surface too early?
Ideally the body should expel its gases before the glaze melts. Modern fast fire frits are specially formulated to melt much later. The modern whiteware industry is build on this premise and glaze formulations have been completely transformed in recent times. Fusion frit 300 is an example. If you are using early melting high boron frits reformulate your glaze to take advantage of fast fire formulations even if you don't fast fire.
Is there a problem with glaze application?
If a glaze layer is too thin pinholes may be a product of a simple lack of glaze to heal them. Increasing the glaze thickness may dramatically reduce the pinhole population (of course your glaze must be stable enough not to run if applied thicker and it must fit well enough not to start crazing due to increased tension between it and the body). Keep in mind that what may appear to be pinholes may actually be blistering, this is often evident when increased glaze thickness reduces the pinhole count but reveals the remnants of many healed blister craters (dough nut shaped rounded bumps on the surface when viewed at an angle in the light).
It is possible that improper application could contribute to pinhole formation. Such pinholes will usually be larger and possibly not be true pinholes, and they may be accompanied by crawling. To deal with this make sure your glaze slurry does not have too much water, that it lays down into a dense layer on the body and that it bonds well to produce a homogeneous dried surface with minimum airspace. To encourage the production of a good surface during drying make sure ware is clean and dust free and that glaze does not form pinholes during drying (try prewetting the ware slightly if the latter happens). Many companies deflocculate their glazes to get a denser lay down.
Is the glaze contaminated?
If pinholes are isolated and few in number it may be possible that a contaminant is getting into the glaze. Pour a sample through a fine screen to check. Do not underestimate the value of ball milling to improve fired glaze surface qualities, many a problem with pinholing and blistering has been solved this way. Many companies ball mill up to 12 hours for best results.
Is the ware once-fire?
Once-fired ware is much more prone to crawling and pinholing because the glaze-body bond is more fragile after application and much more gas is generated during firing than for a body that has already be bisquit fired. Thus, while crawling is the most frequent complaint in once-fire glazed ware, pinholes are more common because of the significant out gassing associated with first-fire. If you add fast-fire to this mix sometimes it is a wonder that it is even possible to get a nice fired surface on a glaze! Try bisque firing to see if this eliminates the problem. If it does then the gases of firing a raw body are not being passed by your glaze; reassess the whole process to reduce all contributing factors as much as possible. Use a fast-fire glaze. See the article on blisters for related information.
Is there problem with the glaze firing?
If ware is fired too rapidly the glaze melt may not have a chance to smooth over. If thicker or protected sections of ware have more pinholes this is usually an indication that slower more even firing will improve the surface over the entire piece. Also, if glaze does not pit or pinhole in sections opposite an unglazed surface that it is clear that body gases are the problem and firing needs to be compensated at the right time (of the body needs to use cleaner materials).
You need to consider both the needs of the glaze and body to determine where in the firing curve to fire more slowly. In most cases non-fast-fire settings fire slower toward the high end (i.e. an hour per cone at cone 6), soak if possible, and slow the initial cooling phase. If the glaze contains an early melting material (i.e. a high boron low alumina frit) you may need to slow the firing just before the frit begins to fuse to allow as much gas to vent as possible before continuing. Most frit suppliers supply melting or softening temperature information.
Modern automatic kiln firing devices make it very easy to control the firing curve. Serious pinholing problems have often been completely eliminated after studying the gas evolution characteristics of body and glaze and employing a firing curve that slows down at appropriate times. Many engineers in industry specialize in the study of firing curves and the programming of automatic kilns. For an example of a TGA (thermal gravimetric analysis) curve, see Copper Carboante and Copper Oxide on this site).
A very important factor to consider also is that modern industrial kilns supply a lot of airflow to the chamber and this carries away products of decomposition. If you are using a kiln without adequate ventilation then there may be not be enough oxygen available at the glaze surface to oxidize and carry away the carbon products of decomposition. Ventilation systems can be added to kilns but that does not mean they are adequate, the air may not be passing over all sections of the ware or at a great enough rate. Some industrial kilns have so much airflow that taller ware can actually blow over if it is not set correctly! If you are doing fast-fire this is critical, a fast fire kiln absolutely must have good air flow. If you are using an electric kiln without airflow, then expect glaze imperfections unless you are firing very slowly.
Another factor to consider is that surface pitting can occur even on cool down (e.g. high sulphur bodies). Thus you may need to adjust the kiln firing program to cool more slowly until the glaze stiffens.
Is the problem with the bisque firing?
Since most pinholes are the product of escaping gases, it is logical to bisque as high and as long as possible to eliminate the bulk of gases during that firing. The only disadvantage of bisquing higher is that ware will be less absorbent and thus may not be as easy to glaze. Find a good compromise temperature. Also, do not stack ware too tightly in the bisque and make sure there is good airflow in the kiln.
It is important that the bisque fire be conducted in an oxidation atmosphere. If not Fe2O3 within the body may be reduced to FeO, a strong flux. During the glaze firing an active glass will be formed within the body and the associated decomposition processes will generate gases that may cause bloating, blistering, or pinholing.
Is the problem spit-out?
If the surface of the glaze is covered with minute broken blisters then the problem is probably spit-out, a condition caused by expulsion of trapped water vapor inside porous ceramics on refire for luster decoration. It is amazing how long it can take to drive off all the water in a fast firing, it may still be coming off past red heat! Make sure the ware you put in a glaze firing kiln is dry.