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Clay | Glaze
| Firing | Toxicity
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| Glossary
'Pinholes' are small holes in the fired glaze surface penetrating down to the body below, often into a surface pore or opening. They 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 been similar to those of blistering. Small pinholes may actually be crawling instead.
When pinholes occur there are often more than one contributing factor, although generally a true pinhole is a problem with the body. Still most strategies to eliminate pinholing involve attack on several fronts, including things like reducing burn-off by higher bisque or cleaner bodies, distributing outgasing by finer grinding of the body, either giving the gases time to escape by slower firing or using a fast-fire glaze that melts later, giving the glaze time to heal by soaking, 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 laydown of glaze.
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.
If you can see 'white spots' and dimples on the glaze surface this suggests other pinholes have healed. It points to outgasing in the body. These spots 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. If talc is being used in small amounts you might try using a frit as a body flux instead.
Are the particles melting vigorously?
Use a sieve to isolate some of the coarser particles and fire them to body temperature. Note if any of them are active melters. If so examine a pinhole 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.
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 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 body too open?
Does the fired body have an open porous structure resulting from many coarser particles or laminations and air pockets resulting from poor pugging (i.e. sand, grog, shale, unground clay)? If pores are networked then gases escaping from within are channelled 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 bisque surface rough or irregular?
If the body surface is rough (because the body contains grog or sand, or the ware has been mechanically trimmed during leather hard stage, pinholes often occur as the glaze dries on the body; these may not heal during firing). In addition, a rough surface exposes pore networks inside the body to larger volume 'exit vents' where greater volumes of gases escape during firing. 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 glaze fill surface irregularities rather than compress air in the voids then have it blow back out as a pinhole a minute later.
Do you use binders?
Glaze binders have been known to produce serious pinholing 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.
Are any glaze materials contributing to the problem?
Some glaze materials produce large volumes of gases as they decompose during firing (Whiting, Dolomite, Talc). These materials can decompose as late at 1000C, if this is after the glaze has started to melt it means trouble. However this usually produces glaze blistering instead and 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.
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 good text will say for sure.
Increasing flux content to produce a more fluid melt often works well to combat pinholes. 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. 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 like to cause blisters. 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.
The thicker the glaze layer the longer the journey for each bubble before it can break at the surface and the larger the individual bubbles tend to be. Applying a thinner glaze layer may result in smaller bubbles with less distance to travel. However be careful not to get the glaze layer too thin or pinholes could result from lack of glaze to heal them.
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 dense laydown.
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.
Once-fired ware is much more prone to crawling and pinholing because the glaze-body bond is more fragile after application. Thus, while crawling is the most frequent complaint in once-fire glazed ware, pinholes often result also because of the significant outgasing associated with first-fire. Try bisque firing, 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.
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.
Depending on the glaze you need to fire more slowly at different stages. In most cases 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 and serious pinholing problems have often been completely eliminated after adding one of these devices to control kiln 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.
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.
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.
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