Small open craters in an otherwise sound glaze are usually called pinholes. They can be no larger than a needle prick or broad enough to catch a fingertip, and they often appear where the glaze is thickest. The defect is disappointing, but it is also useful evidence: something escaped from the clay or glaze while the surface was already beginning to seal.
The most reliable remedy is not simply to add more glaze. Begin by identifying the defect, then work through the clay surface, bisque, application, drying, and firing in that order. A few quiet changes can make a marked difference.
First, identify the surface defect
Pinholes are small, sharp-edged openings through the glaze. The surrounding surface may be glossy and well melted. Some are visible as dark dots; others show as tiny craters when the pot is held obliquely to the light.
A blister is usually larger and more rounded. It forms when a bubble rises through the glaze and leaves a swollen or broken bubble-like mark. A crater may have a more pronounced hollow, sometimes with a raised rim. Crawling is different again: the glaze has pulled away in broad islands, exposing the clay beneath. It is often associated with dust, grease, an overly thick application, or poor adhesion.
These faults can occur together, but the distinction helps. A pinhole usually asks for better gas release or a longer period in which the glaze can heal. Crawling asks you to examine cleanliness and adhesion first. If the glaze is crawling, adding a slower cooling cycle alone will not solve the underlying problem.
Photo by Manki Kim on Unsplash.
Prepare the clay surface before bisque firing
The glaze can only smooth over the surface it receives. A body containing coarse grog or sand may leave open texture in which air and firing gases collect. Trimming can also tear small pores into the clay, especially if the tool drags across a leather-hard surface.
While the pot is still leather hard, soften rough trimmed areas with a rib, a damp sponge, or another method suited to the form. Do not scrub away detail that is meant to remain, but remove loose crumbs and sharp torn pores. A fine-grained slip can sometimes provide a smoother foundation where the body itself is unusually open or rough.
Organic material and moisture in the clay body must leave during firing. For that reason, dry greenware slowly and thoroughly before it enters the kiln. A pot that feels cool may still contain moisture in its thicker parts. Keep walls, handles, joins, and the base in mind rather than judging only the outside surface.
Give the bisque a useful foundation
A sound bisque should be firm, evenly fired, and porous enough to accept the glaze. If the bisque is too soft or uneven, the glaze may settle inconsistently. If the bisque and glaze firings are very close in temperature, the clay may continue to release gases during the glaze firing, just as the glaze is becoming fluid and less able to heal.
Use witness cones alongside the kiln’s programmed temperature when possible. They show the heat work received by the ware rather than relying on the display alone. Follow the firing range recommended for the clay and glaze combination, and avoid assuming that a familiar cone number behaves identically in every kiln.
Before glazing, remove dust from the bisque. A clean, barely damp sponge can help, or the ware may be washed briefly without being soaked. Allow it to dry completely afterward. Do not handle the surface with greasy hands, and keep wax, sanding residue, and loose kiln dust away from the glaze area.
Apply enough glaze, but not too much
Glaze thickness is one of the first things to inspect. Thick glaze holds a larger molten layer in which gases can become trapped, and drips or heavy pools near the foot are frequent sites of pinholing. Thin glaze, however, may fail to mature or may show the body beneath. The aim is an even coat suited to that particular glaze, not the greatest possible coverage.
For dipped ware, keep the movement steady and allow excess glaze to drain. For brushed ware, lay on even coats and let each coat lose its wet shine before adding the next. Repeated brushing over a partly dry surface can disturb the layer, compact air into it, or lift glaze from the bisque.
Clean the foot and any areas that must remain unglazed. A wax pencil can mark a boundary before application, but wax should not be allowed to contaminate the glaze surface. Where the form changes direction, watch for runs and pools. A small amount of glaze gathered in a shoulder, inside corner, or near the base may be enough to create a cluster of defects.
If a test piece shows pinholes only in its thickest areas, reduce the application slightly before changing the entire firing schedule. If pinholes cover a thin, even coat, look more closely at the bisque, clay body, glaze condition, and kiln cycle.
Let the glaze dry and settle
A glazed pot should be completely dry before firing. Dampness is not the only concern: the glaze layer also benefits from being left undisturbed after application. Carrying, tapping, or brushing the ware while the coating is still fragile can introduce small cracks and bare patches that later resemble firing defects.
Stir the glaze thoroughly according to its instructions and keep settled material from concentrating at the bottom of the bucket. If the glaze has been used repeatedly, check that it has not accumulated scraps of bisque, brush hairs, wax, or other debris. Straining may be appropriate for some studio glazes, but use a mesh and method compatible with the glaze’s ingredients and intended texture.
Never correct a dry, dusty surface by flooding it with a heavy second coat unless the glaze maker’s directions support that approach. A light, controlled adjustment is safer than burying the first layer beneath more material.
Photo by Viviane Okubo on Unsplash.
Adjust the firing rather than simply firing hotter
Fast firing can leave too little time for gases to escape before the glaze becomes fluid. A slower approach through the parts of the firing where the clay and glaze release gases may help. Good ventilation and loading practices also matter, so follow the kiln manufacturer’s guidance and avoid overcrowding the chamber.
A hold near the end of the glaze firing sometimes gives small openings time to close, but it is not a universal cure. Holding too long, or firing beyond the glaze’s proper range, can produce running, excessive gloss, color change, or new blisters. Test any schedule adjustment on a tile or a small representative piece first.
A slower cooling period may also help a glaze surface finish healing before it becomes fully rigid. The correct schedule depends on the clay, glaze, kiln, and desired surface. Change one factor at a time and record the result. Witness cones are valuable here, particularly when the kiln’s controller and the ware appear to disagree.
If a bisque firing and a glaze firing are close in temperature, consider whether the bisque is allowing enough of the body’s gases to leave beforehand. The answer is not simply to increase every firing: the bisque and glaze must be treated as a pair, with the clay body’s instructions and the glaze’s firing range in view.
A practical diagnosis guide
| What you see | First things to inspect | Sensible first adjustment |
|---|---|---|
| Pinholes in thick areas or drips | Glaze thickness and pooled application | Apply a little less and even the coat |
| Pinholes across a rough body | Trimmed surface, grog, dust, open pores | Smooth the greenware and clean the bisque |
| Pinholes on many pieces from one batch | Glaze condition and firing schedule | Check mixing, contamination, ventilation, and heat work |
| Rounded bubbles or broken blisters | Trapped gas and a glaze that sealed early | Slow the relevant firing stage and test a hold |
| Broad bare islands | Dust, grease, wax, or excessive thickness | Improve cleaning and adhesion before changing firing |
Keep notes for each firing: clay body, glaze batch, application method, coat count, kiln position, cone readings, and visible defects. Patterns become easier to see when the variables are written down. If only the pieces near a particular shelf or wall are affected, kiln atmosphere, heat distribution, or local thickness may be involved rather than the glaze recipe alone.
Repairing a fired piece
Small, shallow pinholes can sometimes be repaired by carefully smoothing the rough surface, cleaning away all dust, applying a compatible thin coat, and refiring within the glaze’s recommended range. Use suitable respiratory protection and control dust; wet finishing is preferable to creating a dry cloud of glaze or clay particles. Stop before the clay body is exposed more than necessary.
A refire is not guaranteed. Existing holes may remain, and a hotter or poorly chosen refire can enlarge blisters, change the color, or create further pinholes. A cooler refire may be appropriate for some glaze systems, but only when it remains compatible with the glaze and the original firing. Test on a comparable piece whenever possible.
For functional ware, inspect repaired surfaces carefully. A rough pinhole can hold residue and make cleaning difficult. If the defect is deep, widespread, or located where food and liquid will contact it, setting the piece aside for decorative use may be the more sensible choice.
Photo by Wendell Shinn on Unsplash.
The most dependable order of work
When preventing pinholes, follow this sequence: smooth the clay while it is workable; dry the ware fully; fire a consistent bisque; clean rather than soak the bisque; apply an even, moderate glaze coat; let it dry undisturbed; then fire with enough time for gases to escape and the glaze to heal.
Change only one or two conditions at a time. Pinholes are often the result of several small weaknesses meeting at the kiln, and a measured process makes the cause easier to find. With clean surfaces, restrained glaze thickness, and a firing schedule suited to the materials, the glaze is given the quiet opportunity it needs to become smooth.