Though it seems easy, glazing can often be more frustrating than you can ever imagine. While glazing, you might face problems like pinholes that appear on the pottery body. Pinholes mostly come out as small holes, which are not much larger than the head or tip of a pin. Most of the time, pinhole potteries lead to a faulty result that destroys the entire appearance of the piece.
Not only beginners face this issue, but sometimes the experienced potters also face this problem because of their mistakes in the glazing process. So, if you are thinking of how to get rid of such glazing related issues, then you are here at the right place.
In this article, we are going to talk about how easily you can fix all those glazing problems that lead to pinhole formation on the pottery surface. But before that, you must have a good idea about what are pinholes and the reasons behind their formation. So let’s start with it, without buzzing around anymore.
What are pinholes?
Pinholes refer to the small holes usually found on the pottery surface after glaze firing that most of the time penetrates down through the body. With not more than the size of a pin, it appears on the pot surface as a bunch of holes, which occurs due to the roughness present in the clay body, insufficient control of the firing cycle, faulty glaze composition, and many more.
However, often pinholes get mistakenly confused with pits, which looks almost similar. So, to solve the issue, you need to know the difference between the two, as stated below.
Pinholes are usually small holes that appear on the pottery body post glazing, which penetrate down the body below.
Pits are even smaller than the pinholes that appear on the clay body after glazing. But pits don’t penetrate down the pottery body below.
Pinholes are often caused due to the generation of gases from the decomposition of the organic materials present in the glazing mixture or escaping of the crystal water.
Pits are often caused by the air bubbles trapped inside the clay body, which tries to escape after the glaze melting.
What are the reasons behind pinhole formation & how to solve it?
Well, there is more than one reason that contributes to making the glaze look like a faulted one. Here are some of the reasons that lead to pinhole formation on the pot surface along with their solutions.
Eliminate the presence of large or gas-producing particles:
Most of the time, large particles stay trapped in the glazing mixture that causes pinholing post glaze firing. The presence of compounds like lignite, sulfur, etc. eliminates the healing capabilities of the glazing material and contributes to creating such glazing mistakes.
You can quickly fix this problem by following a few easy steps. To know if there is any large compound present in the glazing mixture, start with weighing it, then fire the clay body to cone 04, and after that, again weigh a minute amount of the clay material. If it weighs more than the specified threshold level, then it’s due to the presence of large volatile particles in the clay pot. And this leads to problems like gas trapping, which works as one of the initial reasons behind pinholing.
So, to get rid of this problem, either remove the large particles from the clay body to get the gas distributed in less volume or go on using clear clay with low-sulfur and low-ignite. You can consider using Sargent Art Plastilina Modeling Clay as it has a low-sulfur content and discourages pinholing during glazing.
Look for the presence of double salts in the body:
If you find out that the barely fired clays are creating a glassy film, then it must be because of the soluble salts that move out of the body during the drying process of the pot. If soluble salts get high in fluxing oxides, then it contributes to the pinhole formation in your pottery.
To solve this problem, you need to add some barium carbonate while mixing the clay. It will result in the precipitation of the soluble salts or their substitutes present in the clay body.
Avoid rough bisque firing:
Most of the time, the clay body contains grog or sand on its surface, which forms pinholes on the pottery surface. And an immature or under-fired bisque aggravates this problem by turning the raw pinholes into fired ones.
If the kiln’s firing schedule is too short, then eventually it is going to leave the clay pot incompletely fired. Most of the time, it is the inadequate escape of the gases while bisque firing that works as the anchor of the reasons behind pinholing. And the pottery clay can eliminate these gases only if you bisque fire it for a long time at a high enough temperature. But consequently, this action makes the ceramic less absorbent, which causes problems during glaze application.
So, to avert this problem, all you need to do is find the right temperature to bisque fire your clay, and that too in an oxidation atmosphere.
Solve the problems during glaze firing:
Too much rapid firing doesn’t let the glaze set on the pottery body properly. And this causes the glaze to have a not-so fluidic consistency, preventing the clay from healing itself. It further leads to the generation of too many gases from the clay body, causing pinhole formation.
To solve such problems, you need to make the glaze firing process more accurate and free from any mistake. For that, you need to increase the maturing temperature of the glaze so that your glazing compounds can attain proper glazing maturation. Also, it is essential to hold the kiln at the glaze maturing temperature for a longer time and let the glaze sit for 2 hours or more so that it sticks on to the pottery surface properly.
Hold yourself back from making a mistake while mixing the glazing compounds:
Most of the time, if the glazing ingredients are not appropriately mixed, then it is undoubtedly going to create problems post-application. If the glazing mix is thick enough or zinc and rutile content are high in the mixture, then it is going to thicken the glaze applied on the surface of the pot.
So, to solve this problem, you need to add more flux to the glazing mixture and make it more fluidic so that the compounds present in the glaze reach maturation during firing. Besides, reducing the amount of zinc and rutile present in the glazing mixture also helps in preventing the glaze from being too thick or avoids unwanted fusing.
If you are considering using zinc in the glazing mixture, then try calcining it to make the blend more adaptable to firing.
Some of the glazing ingredients are also responsible for pinholing on the pot body. Here we have enlisted some of those ingredients so that you can avoid using them in your glazing mix.
How do they cause pinholing, and what are the solutions to avoid it?
Using too much glaze binders while mixing the glazing components leads to pinholing and pitting. To avoid this, stay away from applying binders or use bentonite as a substitute. Also, you can interchange the binder that you are currently using with another one, which would aptly suit your glaze.
Dolomite, talc, whiting, copper, cobalt
These compounds promote late decomposition of the glaze while firing. It begins at about 1000℃ temperature, which is attained much after glaze melting, and this results in pinholing. So, consider replacing such compounds with oxide sources like wollastonite, frits, and coloring oxides.
However, if you are planning to use a glaze binder, then make sure you are using a good one. You can use Ceramic Supply USA — Binder for Enamel Powders to get the best results and avoid pinholing. Also, while whisking the mixture, do make sure there are no air bubbles present in it.
Avoid improper application of the glaze:
Pinholes can appear if the glaze is not appropriately applied to the clay pot before firing. In such a case, it leads to uneven maturation during firing, which is another reason behind creating faulty pottery.
The too thick application of the glazing mix is not at all good, and it gives an improper texture to the pottery body, making the pinholes appear on its surface.
Thus, to avoid thick glaze application, you need to make sure that the glazing mixture is not much dense and has a creamy milk-like consistency, which doesn’t thicken up with the application and evens out chemical fusing during firing.
Smooth out the rough clay surface after bisque firing:
If the clay body contains too rough a surface that has numerous pores or vents on it, then it will eventually help the air to get trapped inside it or start to exit during glazing, forming pinholes.
Therefore, to solve this problem, you can either rub the pottery surface along with sandpaper to smoothen it out or opt for trimming the pot when it is in a leather-hard state. Also, applying a fine-grained slip helps much. Washing the bisque-fired pottery can make the wet glaze fill all the irregularities during the application of the glazing and, thus, avoid pinholing problems caused due to blowing back of the air that was compressed in the clay body.
Stay away from contaminated glaze that often results in pinholing:
Often your glaze might get contaminated with the presence of unwanted pollutants in the glazing mix. While making the glazing mixture, make sure you are not using any toxic ingredients or tools.
You can use the technique of ball milling to make your glaze stay uncontaminated for a long time. Using a ball mill can easily give you significant results and prevent pinholing and other glaze contamination problems. Also, clean the glazing tools after every use to avert contamination.
Replace the too much porous clay body:
If your clay pot is too porous, then it tends to create more air vents to let the gas escape through the network channeled in the body during glazing; and this eventually results in the creation of pinholes.
This open porous structure of the pottery body is the result of the presence of coarse particles on its clay surface. It usually occurs due to the low maturation during bisque firing. And for that, you need to bisque fire it at the right temperature.
Always try to find a clay body with finer particles for glazing. You can also use a fine slip to apply on the pottery body before glazing to lessen its porosity and make the clay pot smooth enough to glaze properly.
Proper understanding of the gas evolution process:
Gas evolution and the emission of gas in a bulky way during bisque firing or glaze firing is one of the main reasons behind pinholing.
To understand the gas emission process, you need to have adequate knowledge about it. And for that, you need to have a good observing power during the bisque firing and glaze firing process. To control this too much of gas emission, you need to slow down the firing process to reduce the evolution phase.
Avoid too early glaze melting:
The gas present in the glazing mix needs to be expelled before the glaze starts melting. But if the melting happens too early, the gas will try to get emitted out of the clay body creating small holes on the sealed pottery.
Most of the modern frits come with late melting characteristics so that the gas can come out of the body adequately before the glaze starts to melt. If you are using frits that contain early melting boron elements, then it will begin melting too fast even if you do not intend to fire it quickly.
So here, we have discussed most of the common problems that result in pinholing on a glazed pottery body. If you are encountering frustrating problems during glazing, use any of the tips mentioned above to make your result effective. As a beginner, you must learn the characteristics of glazing appropriately so that you can easily understand the gas evolution and air escaping mechanisms to avoid glazing issues. We hope this article will help you to guide you with the pinholing problem.
Originally published at https://spinningpots.com on October 3, 2020.