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Pinholing

Pinholing is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production.

Key phrases linking here: pinholing, pinholes, pinhole - Learn more

Details

A glaze defect where tiny holes are present in the fired glaze surface. These holes normally go down to the body surface below. Pinholing is a plague in industry, the tiniest hole in the glaze surface of a tile or utilitarian item can make it a reject. Industry goes to great pains to get materials of very fine particle size for their bodies and glazes to reduce the occurrence of glaze defects. They need to be sure that ware can survive their fast firing schedules.

Glazes that melt and flow well often still have pinholes if gas producing particles are present in the body (these expel gases up through the glaze melt thereby disturbing its surface). Thicker layers of glaze pinhole more than thinner ones. Blisters, dimples and pinholes often occur together. Fast firing produces more pinholes.

Potters and smaller operations have a universal solution that manufacturers do not: The freedom to use drop-and-hold and slow-cool firing schedules, these can produce a perfect glaze surface on almost any clay body. Reduction gas firing is known to produce much fewer glaze defects, the reason is often that the kiln, because of its mass, cools much slower.

Some glaze chemistries are much more prone to pinholing. For example, high Al2O3 and ZrO3 stiffen the glaze melt making it less able to heal defects.

Related Information

Let me count the reasons this glossy white cone 6 glaze is pinholing

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First, the layer is very thick. Second, the body was only bisque fired to cone 06 and it is a raw brown burning stoneware with lots of coarser particles that generate gases as they are heated. Third, the glaze contains zircopax, it stiffens the melt and makes it less able to heal disruptions in the surface. Fourth, the glaze is high in B2O3, so it starts melting early (around 1450F) and seals the surface so the gases must bubble up through. Fifth, the firing was soaked at the end rather than dropping the temperature a little first (e.g. 100F) and soaking there instead.

Same glaze/body. One fired flawless, the other dimpled, pinholes. Why?

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The difference is a slow-cool firing. Both mugs are Plainsman M340 and have the L3954B black engobe inside and partway down on the outside. Both were dip-glazed with the GA6-B amber transparent and fired to cone 6. The one on the right was fired using the PLC6DS drop-and-hold schedule. That eliminated any blisters, but some pinholes remained. The one on the left was fired using the C6DHSC slow-cool schedule. That differs in one way: It cools at 150F/hr from 2100F to 1400F (as opposed to a free-fall). It is amazing how much this improves the brilliance and surface quality (not fully indicated by this photo, the mug on the left is much better).

Solving a pinholes problem with a boron fluxed cone 04 opacified and stained glaze

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This problem was suffered by a potter moving from Europe to Canada. In Europe she used lead based glazes and got smooth defect-free surfaces. But in Canada she began using our Gerstley Borate based glazes and had many problems adapting to them. We used glaze chemistry to both adjust recipes to source boron from frits instead of GB and to reduce their melt fluidity. She also adjusted firings: Higher bisque and glaze firings and a drop-and-hold firing schedule (like C03DRH). And she adopted a finer particle-sized terra cotta clay body. These measures enabled her to eliminate the issue.

Pinholes often happen on trimmed surfaces

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Pinholing is often related to the smoothness of the underlying body surface. The lower half of this vase was tooled during the leather hard stage, all the pinholes occur there. Even though this body contains 10% grog, the pinholes are not appearing on the upper half because the slip generated during throwing has left a smooth surface.

How can you best tell if a base glaze is prone to pinholing with your body?

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Testing for pinholes and dimples is often best done using a transparent glaze over a large surface and looking at the surface in the light. In this case, an open bowl is used. Heavy pieces like this are difficult to fire evenly and encourage under fired areas where pinholes are more likely to appear.

What if you just cannot solve a pinhole problem?

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Pinholing on the inside of a cone 6 whiteware bowl. This is glaze G2926B. The cause is likely a combination of thick glaze layer and gas-producing particles in the body. Bodies containing ball clays and bentonites often have particles in the +150 and even +100 mesh sizes. The presence of such particles is often sporadic, thus it is possible to produce defect-free ware for a time. But at some point problems will be encountered. Companies in large production need to have fast firing schedules, so they either have to filter press or wet process these bodies to remove the particles. Or, they need to switch to more expensive bodies containing only kaolins and highly processed plasticizers. But potters have the freedom to use drop-and-hold or slow-cool firing schedules, that single factor can solve even serious pinholing issues.

Oversize particles in a typical manufactured porcelain body

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Example of the oversize particles from a 100 gram wet sieve analysis test of a powdered sample of a porcelain body made from North American refined materials. Although these materials are sold as 200 mesh, that designation does not mean that there are no particles coarser than 200 mesh. Here there are significant numbers of particles on the 100 and even 70 mesh screens. These contain some darker particles that could produce fired specks (if they are iron and not lignite); that goodness in this case they do not. Oversize particle is a fact of life in bodies made from refined materials and used by potters and hobbyists. Industrial manufacturers (e.g. tile, tableware, sanitaryware) commonly process the materials further, slurrying them and screening or ball milling; this is done to guarantee defect-free glazed surfaces.

Lignite contamination in manufactured porcelain body

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These contaminating particles are exposed on the rim of a bisque-fired mug. The liqnite ones have burned away but the iron particle is still there (and will produce a speck in the glaze). Remnants of the lignite remain inside the matrix and can pinhole glazes. Since ball clays are air floated (a stream of air takes away the lighter particles and the heavier ones recycle for regrinding) it seems that contamination like this would be impossible. But the equipment requires vigilance for correct operation, especially when there is pressure to maximize production. Ores in Tennessee are higher in coal than those in Kentucky. North American clay body manufacturers who confront ball clay suppliers with this contamination find that ceramic applications have become a very small part of the total ball clay market - complaints are not taken with the same seriousness as in the past.

Cone 6 glazes can seal the surface surprisingly early - melt flow balls reveal it

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These are 10 gram balls of four different common cone 6 clear glazes fired to 1800F (bisque temperature). How dense are they? I measured the porosity (by weighing, soaking, weighing again): G2934 cone 6 matte - 21%. G2926B cone 6 glossy - 0%. G2916F cone 6 glossy - 8%. G1215U cone 6 low expansion glossy - 2%. The implications: G2926B is already sealing the surface at 1800F. If the gases of decomposing organics in the body have not been fully expelled, how are they going to get through it? Pressure will build and as soon as the glaze is fluid enough, they will enter it en masse. Or, they will concentrate at discontinuities and defects in the surface and create pinholes and blisters. Clearly, ware needs to be bisque fired higher than 1800F.

Mortified to think of the amount of pinholing I once tolerated as normal

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A cone 6 blue silky matte glazed vase with thousands of pinholes in the glaze

This cone 6 vase was made using a coarse-grained stoneware. That generates lots of gases on firing, and it left behind plenty of pinholes as a testament. Today I still use clays like this but my pieces have almost no pinholes. Why? Electronic kiln controllers enable me to do drop-and-hold and slow-cool firings. It is that simple. Well, actually, there is another reason: I make sure my glazes have adequate fluidity to be mobile enough to heal the holes during hold, but not so fluid that they percolate while doing that (producing blisters and micro-bubbles).

Inbound Photo Links



Why is this cone 10 oxidation iron-brown glaze pinholing?


Same cone 6 glazes. Same clay. Why is the one on the right pinholing?

A chart showing weight-loss vs firing temperature for common ceramic materials
LOI is not important? Think again!

And Insight-Live screenshot
What can you do using glaze chemistry? More than you think!


One small pinhole in a terra cotta mug and we have a problem


Pinholing at a cone 6 stoneware mug. Why?


Encapsulated stains are firing with a mass of bubbles and pinholes

Links

Troubles Glaze Pinholes, Pitting
Analyze the causes of ceramic glaze pinholing and pitting so your fix is dealing with the real issues, not a symptom.
Media A Broken Glaze Meets Insight-Live and a Magic Material
Use Insight-Live.com to do major surgery on a feldspar saturated cone 10R glaze recipe with multiple issues: blistering, pinholing, crazing, settling, dusting and possibly leaching!
Media Manually program your kiln or suffer glaze defects!
To do a drop-and-hold firing you must manually program your kiln controller. It is the secret to surfaces without pinholes and blisters.
Glossary Drop-and-Soak Firing
A kiln firing schedule where temperature is eased to the top, then dropped quickly and held at a temperature 100-200F lower.
By Tony Hansen
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