What Causes Pinholes in Pottery Glaze and How To Prevent Them

Pinholes are one of the most common imperfections that occur during or after glazing pottery. Although you may prefer to have a few pinholes for beauty and stylistic reasons, you will also want to minimize them for safety and hygiene. In order to do so, you need to understand what causes pinholes in pottery and how you can prevent them.

The main cause of pinholes in pottery is air bubbles created by gases in the Glaze or Clay body. You can minimize them by using finer particles, adding more flux, applying a second, thinner glaze layer, and using a well-ventilated kiln.

In this post, I will elaborate on the factors leading to pinholes in pottery glaze and what you can do to address them.

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Gases Have Formed in the Glaze

The most common cause of pinholes in pottery is the gases that form on the Glaze in the shape of bubbles. During firing, gases escape from the Glaze and or Clay body and form bubbles, penetrating the surface. 

These bubbles burst and make crates inside the Glaze, known as pinholes. These gases can have many different sources, including organic matter in the clay.

How To Prevent

To make sure the body composition of the clay isn’t the culprit. You can slow your bisque fire down. Raise your bisque firing temperature (no higher than cone 04). Or select a lighter Clay body. Darker clays will produce more gases, while white clays contain less organic materials and burn cleaner.

You will need to eliminate clumps or coarse particles in your glaze by sieving (running it through a strainer). This is especially important when making dipping glazes.

The Glaze Wasn’t Applied Properly

One of the most important features of the Glaze you use is that it should be suitable for the glazing process. For example, if you use a fast-fire glazing process, you need to use the glaze composition that fits this process. You need to add materials that produce a later melt. 

Otherwise, fast-firing using materials not specifically formulated for this process will lead to pinholes and other imperfections.  

Another problem related to the glaze application happens when you glaze the inside and outside simultaneously. In such cases, the water content absorbed during the process traps the air inside the bisque’s pores. Check out Glazing Pottery Dip, Drip, and Brush | Pottery Glazing Tips for more detailed information on applying pottery glaze.

How To Prevent

To avoid this problem, rinse the bisque, wipe it dry, wait for one hour, and glaze it. This way, you can reduce the amount of trapped air and produce fewer bubbles.

Additionally, you can rub down all visible pinholes after applying the Glaze or add a thin layer of Glaze again after adding some water to it.

The Kiln Lacks Proper Airflow

During the firing process, the decomposition of the clay body and glaze materials produces many byproducts in the form of gasses. You need oxygen to oxidize these products and take them out of the kiln and away from the glaze surface.

How To Prevent

Make sure your Kiln has adequate airflow to let the air pass through all the ware. Keeping the top peep open throughout the firing will help. Also, having a downdraft venting system.

Pinholes can also form during cooldown, which makes it important to manage this process. This is because fast cooling freezes the bubbles before the Glaze has a chance to flow and get rid of the pinhole. Smaller Kilns do cool down faster. Controlled cooling or having thicker bricks helps to slow down the cooling process.

The Single-Firing Process Can Reduce the Body’s Strength

After crawling, pinholes are the second most common problem in single-fired pottery. As the name suggests, once-firing refers to the process of glazing the dried ware in one operation. It makes the bond between the Glaze and the Clay body weak, creating more gas during firing than bisque fired. 

How To Prevent

One possible solution is bisque firing, especially when the gases in the raw clay body don’t pass through the glazing process.

Avoiding bubbles and body dehydration is challenging. You need to make a glaze slurry that tolerates shrinkage, creates a good bond between the body and the Glaze and maintains an efficient drying process. 

The Glaze Is Too Thin

If you add only one layer of thin Glaze, it won’t be able to cover the surface properly. Increasing the glaze thickness or adding another layer of Glaze will help you reduce the number of pinholes. 

The Glaze must be thick enough so that it is stable and doesn’t run down the surface. Keep in mind that if the Glaze doesn’t bond well with the surface, it will lead to crazing.

How To Prevent

To make sure thin Glaze doesn’t lead to pinholes and other problems (like crawling), you will need to control the amount of water in your glaze material. This will ensure that it’s dense enough to stick to the surface and thin enough to run in a homogeneous flow and minimize airspace. 

Keep in mind that the water you use should be distilled and dust-free to avoid making pinholes while the Glaze dries. If the Glaze is contaminated, you may experience sparse and few pinholes. You can check by running the Glaze through a fine screen. 

The Glaze Was Fired Too Quickly

Suppose you perform the glaze firing process too rapidly. In that case, the glaze melt doesn’t have enough time to run over and eliminate the gases. If you see more pinholes in the thicker parts of your pottery pieces, you can fix the issue by firing more slowly.

How To Prevent

The Glaze and clay body’s characteristics determine what part of the process needs a slower firing. For example, when you have early melting materials in your Glaze, it’s better to wait for the frit to start fusing. You can just slow down the firing just before that to allow the maximum amount of gas to go out. 

It’s better to choose the slow rate of speed when firing. Or you can fire at a med rate of speed one cone down and apply a 10 to 20-minute hold at the end of the fire. Again, it’s about the temperature and the time.

For example, if you fire to cone 6. you will want to fire to cone 5 and place a 20 hold at the end. This is equal to firing to cone 6 but allows the glaze to cure more. I glaze fire this way and hardly ever get pinholes.

The Bisque Firing Process Was Too Short

The primary factor in creating pinholes is the production of gases. However, long bisque periods can eliminate these gases. That said, you shouldn’t go overboard with a long and high bisque firing as it may lead to overfiring, another cause of pinholes. In addition, high bisque firing makes the ware less absorbent, reducing the effectiveness of glazing. A recommended bisque temperature is cone 06 and even 04.

How To Prevent

The only solution is to find an optimal temperature between high bisque firing and overfiring by analyzing the clay body and glaze compositions. 

Another thing to remember while bisque firing is to soak briefly at the highest temperature to help the bubbles fire out during the bisque firing. For example, you can fire to Cone 05 or 06 hold at peak temperature for 15 to 20 minutes.

The Glaze Is Too Dense

When you melt the Glaze, it should flow freely to let gas bubbles pass and flow out of the way without getting trapped in the surface. Unfortunately, resistance to flow occurs due to inadequate melting. That said, some materials inside the Glaze can make it too dense. For example, even if the Glaze melts well, high zirconium or aluminum contents can make the Glaze not flow smoothly enough.

How To Prevent

Melt flow testers can help you take out the guesswork from determining the melting properties because looking at the glaze flow doesn’t help you decide how fluid the Glaze is.

Suppose the tests show that your glaze fluidity is low. In that case, you can increase the flux levels by adding small amounts of zinc oxide, lithium oxide, or strontia. That’s particularly the case when you see vertical ware developing fewer pinholes than horizontal ones. It’s a clear sign that the Glaze isn’t fluid enough.

Alternatively, you could decrease the alumina content to make the Glaze more fluid. Still, it’s not suitable for matte finishes because it will give you a glossy effect.

For more detailed information on Glazing Pottery check out 27 Glazing Tips For Beginners

Summary

Pinholes are one of the most common imperfections that form on the surface of the pottery. There are many reasons for this to happen. The leading cause is usually air bubbles created by the organic matter inside the Clay body and Glaze.

Because of all the different clays, glazes, firing temperatures, and rate of firing it can be hard to pinpoint the cause of pinholes. All of these solutions will help you to eliminate pinholes in your pottery.