Why Are There Pinholes in My Ceramic Glaze? Proven Solutions
Pinholes in ceramic glaze are not random accidents. They are trapped gas bubbles that burst through the molten glaze surface during firing and fail to heal over before the kiln cools.
Every pinhole tells you exactly what went wrong if you know how to read it. This guide covers every cause, the science behind why each one happens, and the specific fix for your clay body, glaze type, and kiln setup.
By the Numbers
Pinholes in Ceramic Glaze – What the Research Shows
Sources: Ceramics Monthly Technical Reports, Digitalfire Reference Library (Tony Hansen), Orton Foundation firing data
What Are Pinholes in Ceramic Glaze?
Pinholes are small crater-like depressions in a fired glaze surface where a gas bubble burst through the molten glass and the hole did not close before the kiln cooled below the glaze’s melting point.
They differ from other glaze defects in one critical way: pinholes are caused by escaping gas, not by crawling, shivering, or thermal expansion mismatch. The gas can come from the clay body, the glaze itself, or the kiln atmosphere.
According to Daniel Rhodes in Clay and Glazes for the Potter, pinholes form when the glaze melt viscosity is too high at the moment the gas escapes. A glaze with lower viscosity at peak temperature allows the hole to heal over before cooling locks the surface in place.
In plain terms: the glaze was stiff like honey when the gas bubble popped through it, not runny like water. Runny glaze heals. Stiff glaze leaves a crater.
What Causes Pinholes in Ceramic Glaze? The 5 Root Sources
Pinholes always come from escaping gas. The source of that gas determines how you fix the problem.
There are exactly five places gas can originate. Each one produces pinholes with a distinct pattern you can learn to recognize.
1. Clay Body Outgassing – The Most Common Cause
This happens because organic matter, sulfur compounds, and carbon trapped inside the clay body burn off during firing and the gas must escape through the glaze layer above it.
This only occurs when the glaze melts and seals the surface before all the gas has finished leaving the clay body – typically between cone 06 and cone 1 in a bisque firing, or in the early stages of a single-fire process.
If your bisque firing is too short or peaks too low (below cone 06), the clay never fully burns out its organic content. Then during the glaze firing, those trapped compounds convert to gas at around 1,650°F (899°C), right as your glaze begins to melt and form a seal. The result is a cluster of pinholes concentrated on thicker areas of the pot.
Fix it by slowing down your bisque firing through the carbon burnout zone. Insert a 30-minute hold at 1,650°F (899°C) during every bisque firing. This gives organic materials time to burn off completely before the clay surface begins to seal.
For dark clay bodies, which contain more organic material and manganese, extend that hold to 45-60 minutes. Dark mid-fire stoneware clays are notorious pinhole producers without adequate burnout time.
2. Glaze Layer Outgassing – Gases From Within the Glaze Itself
This happens because certain glaze materials decompose and release gas during firing – carbonates, sulfates, and hydrates are the primary culprits. Whiting (calcium carbonate) releases CO2. Gerstley borate and colemanite release steam. Copper carbonate releases CO2 and leaves copper oxide behind.
This only occurs when the glaze contains raw carbonate or sulfate materials and the firing schedule does not give these compounds enough time to decompose before the glaze surface seals over.
If your glaze recipe contains more than 10% whiting or significant amounts of raw dolomite, these materials begin releasing CO2 at around 1,500-1,650°F (816-899°C). If the glaze has already started to sinter and seal by that point, the gas has no escape path. The fix is substituting fritted materials or calcining your carbonates before mixing the glaze.
Tony Hansen of Digitalfire has documented extensively that switching from raw whiting to wollastonite (a naturally calcined calcium silicate) or using Ferro Frit 3124 as a calcium source eliminates this entire category of pinholes. The frit has already been melted and quenched – there are no gases left to release.
3. Application Defects That Trap Air
This happens because air bubbles are physically trapped in the wet glaze layer during application and those bubbles become pinholes when they rupture during firing.
This only occurs when glaze is applied too thickly, too quickly, or with a brush or spray gun that introduces air into the wet slurry.
Brushing glazes onto bisqueware too aggressively whips air into the wet glaze. Each brush stroke can trap micro-bubbles. Spraying at too high a pressure atomizes the glaze so finely that it entrains air. Even dipping can trap air if the pot is plunged in too fast and the glaze cannot displace air from textured surfaces evenly.
Fix it by checking your glaze specific gravity. A glaze hydrometer reading should land between 1.45 and 1.50 for most dipping glazes. Thicker glaze (above 1.55) traps more air. For brushing, apply three thin coats rather than one thick coat and let each coat dry fully before the next.
glaze crawling from application problems often shares root causes with pinholes – both start with how the wet glaze layer behaves on the bisque surface before firing even begins.
4. Firing Schedule Problems – Too Fast, Too Short, or Wrong Atmosphere
This happens because the kiln reaches peak temperature and then begins cooling before the glaze has had enough time in a fully molten state for all gas bubbles to rise to the surface and for the resulting holes to heal over.
This only occurs when the firing schedule lacks a soak period at peak temperature, or when the cooling rate through the glaze’s freezing range is too rapid for the surface to level out.
A cone 6 glaze reaches full melt at approximately 2,232°F (1,222°C) when fired at a medium ramp rate. At that exact moment, the glaze is still full of tiny bubbles from the final stages of decomposition. Hold the kiln at peak temperature for 15-20 minutes. This allows every remaining bubble to rise through the thin melt layer and for surface tension to pull the hole closed. Skipping this soak is the single most common firing error among glaze defects that appear after firing.
In reduction firing, pinholes can also form if reduction is introduced too heavily or too late. Heavy reduction above cone 1 produces soot that deposits inside the glaze melt. When the kiln clears toward the end of the firing, that soot burns out and leaves craters. Keep reduction light to medium and begin it no later than cone 012.
5. Kiln Dust and Surface Contamination
This happens because tiny particles of kiln wash, brick dust, or other debris settle on the glaze surface before or during firing and act as nucleation points where gas bubbles preferentially form.
This only occurs when kiln wash is flaking, when the kiln interior is dirty, or when pots are handled with dusty or oily hands between glazing and loading.
Kiln wash that flakes off the shelf above your pot lands on the wet or dry glaze surface. During firing, each tiny particle becomes a site where a bubble nucleates and bursts. The resulting pinhole pattern looks like scattered individual pinholes, not clusters. Fix it by vacuuming your kiln between every firing with a HEPA-filtered vacuum and by applying kiln wash carefully so it does not flake.
For most home studio potters, a regular kiln cleaning schedule prevents this entire category of pinholes without requiring any chemistry changes.
Step-by-Step Guide
How to Diagnose and Fix Pinholes – Step by Step
5 steps · Estimated time: 1 to 2 firings to fully resolve
Identify the pinhole pattern
Clustered pinholes on thick areas point to clay body outgassing. Scattered individual pinholes suggest application or dust issues. Pinholes across all surfaces equally suggest glaze chemistry or firing schedule problems.
Run a controlled test fire
Place witness cones on every shelf level. Fire one test pot with a 20-minute soak at peak temperature. If pinholes disappear, your old firing schedule was too short. If they remain, move to step 3.
Adjust your bisque firing schedule
Add a 30-minute hold at 1,650°F (899°C) in your bisque firing. Fire to at least cone 06 (1,828°F / 998°C), not cooler. For dark clay bodies, extend the hold to 45-60 minutes.
Check and correct glaze application
Measure specific gravity. For dipping glazes, target 1.45-1.50. For brushing glazes, apply 3 thin coats. For spraying, reduce air pressure and hold the gun farther from the pot surface.
Reformulate the glaze if needed
If pinholes persist after steps 1-4, the glaze formula needs changing. Swap raw whiting for wollastonite or a frit. Reduce zinc oxide below 3% if present. Add 1-2% bentonite to improve application suspension.
How to Tell Which Type of Pinhole You Have
Use the table below to match your pinhole pattern to the correct cause before attempting any fix.
| Pinhole Pattern | Most Likely Cause | Test to Confirm | Primary Fix | Time to Resolve |
|---|---|---|---|---|
| Clustered on thick areas, rims, and bottoms | Clay body outgassing | Switch to a lighter clay body for one firing | Extend bisque hold at 1,650°F | 1 firing |
| Evenly scattered across all surfaces | Glaze chemistry or firing schedule | Add 20-min peak soak | Soak at peak or swap carbonates for frits | 1-2 firings |
| Visible only on one side or specific area | Application defect or contamination | Re-glaze with careful technique | Reduce glaze thickness or clean kiln shelves | 1 firing |
| Pinholes with sharp, jagged edges | Fast cooling or early shutdown | Check pyrometer calibration | Slow cooling through 1,400°F range | 1 firing |
| Pinholes only in reduction firing | Soot burnout from heavy reduction | Fire same glaze in oxidation | Lighter reduction, earlier start | 1 firing |
Preventing Pinholes Before They Happen
Prevention is cheaper than refiring. These five practices eliminate the vast majority of pinhole problems before a kiln is ever loaded.
Master Your Bisque Firing Schedule
The most effective prevention step is a bisque firing that fully burns out all organic material and carbon compounds from the clay body. Fire to at least cone 06 (1,828°F / 998°C), never lower. Cone 04 (1,945°F / 1,063°C) is even better for dark or heavily grogged clay bodies.
Insert a hold of 30-60 minutes between 1,600°F and 1,700°F (871-927°C) in every bisque firing. This single change resolves more pinhole problems than all other fixes combined. Your Orton witness cones should show a complete bend at cone 06 on every shelf before the bisque firing ends.
Control Glaze Application Thickness
A glaze layer that is too thick traps more air and takes longer for gas to escape through. A layer that is too thin may not fully melt into a continuous glass surface. Target 2mm of dry glaze thickness on the bisque surface for most dipping applications.
Push a pottery needle tool through the dry glaze layer to the clay surface to check your thickness. Adjust dipping time or brushing coats until 2mm is consistent across the entire pot.
Soak at Peak Temperature Without Exception
Every glaze firing should include a 15-20 minute soak at peak temperature. This allows the glaze to reach full melt viscosity, for all remaining gas bubbles to rise to the surface, and for surface tension to heal every resulting hole before cooling begins. A soak of even 10 minutes eliminates most pinholes in commercial cone 6 glazes like Amaco Potter’s Choice cone 6 glazes.
Keep the Kiln Environment Clean
Vacuum the kiln interior between every firing. Check kiln wash on shelves and reapply before it flakes. Handle bisqueware with clean, dry hands and store glazed pots under cover where studio dust cannot settle on the unfired surface.
These small habits prevent the scattered, individual pinholes that are most frustrating to diagnose because they appear random and inconsistent from firing to firing.
Choose the Right Glaze for Your Clay Body
Not every glaze works on every clay. A glaze that produces a perfect surface on white stoneware may pinhole severely on a dark, iron-rich clay body because the two materials release different amounts of gas at different temperatures.
Test every new glaze-clay combination on a small test tile before committing to production work. This single habit prevents the worst surprises.
Myth vs Fact
Pinholes in Glaze – Common Myths Debunked
Separating fact from fiction on the most common pinhole misconceptions
✗ Myth
Pinholes mean my glaze was applied too thick.
✓ Fact
While thick glaze can contribute, pinholes are primarily caused by escaping gas – not thickness alone. A properly melted and soaked glaze can be thick without pinholes. Focus on gas sources first, then adjust thickness second.
✗ Myth
Firing hotter will always fix pinholes.
✓ Fact
Over-firing a cone 6 glaze to cone 7 or higher makes the melt too fluid – it runs off the pot and onto the shelf. The correct fix is time at the right temperature, not more temperature. A 20-minute soak at cone 6 achieves proper healing without over-firing.
✗ Myth
Commercial glazes never produce pinholes.
✓ Fact
Commercial glazes pinhole just as easily as homemade ones if the bisque firing was inadequate, the application was sloppy, or the glaze firing schedule lacks a soak. The glaze formula is only one variable in a system that includes clay body, bisque schedule, application method, and firing cycle.
✗ Myth
A pinhole-glazed pot must be discarded.
✓ Fact
You can refire pinhole-affected pots. Apply a thin fresh coat of the same glaze over the pinhole area, then fire again with the correct soak. The new glaze layer fills the craters. For functional ware, refiring also improves food safety by sealing any porosity the pinholes exposed.
✗ Myth
Switching to a different clay body is the only real fix.
✓ Fact
Most pinhole problems are solved by adjusting bisque and glaze firing schedules – not by abandoning your clay body. Only switch clay if you have already optimized both firing schedules and still see persistent pinholes. Start with the free fixes first.
Can You Refire a Pot With Pinholes?
Yes, you can refire pinhole-affected pots. The process requires applying a thin fresh coat of the same glaze over the affected area to fill the craters. Then fire again with the corrected schedule that includes a 15-20 minute peak soak.
The new glaze application must be thin – just enough to fill the holes. Too thick and you may introduce new application bubbles. Let the fresh coat dry completely before loading the kiln.
Refiring works best when the pinholes are shallow and the underlying clay body is fully vitrified. Deep pinholes that penetrate to the clay body may not fill completely because the new glaze cannot flow deep enough into the crater before the kiln begins cooling.
For functional ware intended for food, refiring is recommended because pinholes compromise the sealed glaze surface. Even if the clay body is vitrified and the pinholes do not leak, they trap food particles and bacteria that cannot be cleaned out.
Advanced Glaze Chemistry Fixes for Persistent Pinholes
This section is for potters who mix their own glazes and have already optimized their firing schedules. If pinholes persist after correcting bisque burnout, application thickness, and peak soak time, the problem is in the glaze formula itself.
This happens because certain material combinations in the glaze produce a melt with high surface tension that resists healing. Zinc oxide above 3% is a classic culprit – it increases surface tension in the melt and makes it harder for pinholes to close.
This only occurs in specific glaze types: zinc-based crystalline glazes, high-alkaline glazes with excessive sodium or potassium, and under-supplied silica glazes that form a thin, stiff melt.
If your glaze recipe includes more than 3% zinc oxide, reduce it to 2% or substitute another opacifier like zircopax (zirconium silicate). If your glaze uses raw whiting or dolomite as the primary calcium source, swap to wollastonite or a calcium-bearing frit like Ferro Frit 3124. These materials have already released their gas during manufacturing and will not outgas in your kiln.
Adding 1-2% bentonite by dry weight improves glaze suspension and application uniformity. Better suspension means fewer air bubbles trapped during mixing. For high-surface-tension glazes, adding 0.5-1% lithium carbonate can reduce melt viscosity enough to let pinholes heal without making the glaze runny.
In plain terms: if your glaze is pulling itself into tight balls instead of flowing together, it cannot heal pinholes. Reduce zinc, swap raw carbonates for frits, and add a tiny amount of lithium to relax the melt.
For most studio potters mixing their own glazes, the combination of a 20-minute peak soak and substituting fritted calcium for raw whiting resolves even stubborn pinhole problems without requiring a complete glaze reformulation.
Results
What Changes When You Fix Pinholes Correctly
The transformation from pinhole-glazed to smooth, sealed surface
Before
- ✗Crater-like holes visible across the glaze surface
- ✗Food particles and bacteria trap in unfilled pinholes
- ✗Glaze surface feels rough or sandy to the touch
- ✗Inconsistent results – pinholes appear unpredictably
After
- ✓Smooth, continuous glass surface with no craters
- ✓Fully sealed surface – safe for food and liquids
- ✓Glassy, even texture across the entire pot
- ✓Repeatable results – every firing produces the same surface
The corrected firing schedule plus one glaze chemistry adjustment transforms a rejected pot into a finished, sellable piece.
Frequently Asked Questions About Pinholes in Ceramic Glaze
Why do pinholes only appear on some pots and not others in the same firing?
Quick Answer: Pinholes that appear selectively within a single kiln load point to localized differences in glaze thickness, clay body density, or shelf position. Pots on the bottom shelf often fire cooler – a 20-40°F (11-22°C) temperature difference is enough for pinholes to form on cooler shelves while hotter shelves heal completely.
Check your kiln’s temperature distribution with witness cones on every shelf level. Pots with thicker walls retain heat differently and may reach peak temperature at a different moment than thin-walled pots.
Pots glazed by different people or on different days may have slightly different glaze thickness even if they look the same. Measure specific gravity before every glazing session to eliminate this variable.
Can pinholes in glaze make a pot unsafe for food?
Quick Answer: Yes. Pinholes that penetrate through the glaze to the clay body create pathways for liquids, bacteria, and food particles to enter the clay. Even if the clay body is vitrified (under 1% absorption), pinholes trap organic material that cannot be cleaned out and may support bacterial growth.
For functional dinnerware, a glaze surface with any visible pinholes should be refired or rejected for food use. Decorative pieces and sculpture can tolerate pinholes without functional concerns.
The ASTM C738 standard for ceramic foodware leaching requires an intact, continuous glaze surface. Pinholes violate that continuous surface requirement regardless of the clay body absorption rate underneath.
What is the difference between pinholes and pinholing versus blistering?
Quick Answer: Pinholes are small, fully open craters where a gas bubble burst and the hole remained open. Blisters are raised, dome-like bubbles where the gas pushed the glaze surface upward but did not break through. A blister that bursts during cooling becomes a pinhole with sharp edges.
Blistering indicates more aggressive outgassing – often from over-fired glazes or clay bodies with excessive sulfur. Pinholes indicate milder outgassing where the bubble wall was thin enough to rupture.
The fix for blistering is a longer burnout hold at a lower temperature range (1,500-1,700°F). The fix for pinholes is often a longer soak at peak temperature to heal the already-opened holes.
Why do my clear glazes pinhole more than colored glazes?
Quick Answer: Clear glazes lack opacifiers and colorants that can mask pinholes visually. A pinhole in a clear glaze is more visible because there is nothing to scatter light and hide the depression. Colored glazes may have the same number of pinholes – you just cannot see them as easily against a pigmented background.
Clear glazes also tend to be simpler formulas with fewer flux materials, which can result in higher melt viscosity and less ability to heal pinholes before cooling. Adding 1-2% tin oxide or a small amount of a colored glaze as a tint can help clear glazes flow better and heal more completely.
Do I need to clean my kiln shelves between every firing to prevent pinholes?
Quick Answer: You do not need to clean shelves between every firing, but you do need to inspect kiln wash condition and vacuum loose debris regularly. Flaking kiln wash is the primary shelf-related cause of pinholes – not general dust. Recoat shelves when wash begins to flake at the edges.
A thorough kiln cleaning every 5-10 firings is sufficient for most home studios. Focus on vacuuming the kiln floor and element grooves where debris accumulates. For production studios firing daily, a quick vacuum between loads prevents cumulative dust buildup.
Can I fix pinholes by sanding and applying a thin glaze coat?
Quick Answer: Sanding pinhole-affected glaze to smooth the surface before reglazing does not work well. The sanded glaze surface is too smooth for the new glaze to adhere properly. Instead, clean the pot, apply a thin coat of the same glaze directly over the pinhole area, and refire with the correct soak.
The new glaze coat adheres by absorbing into the bisque at the bottom of each pinhole crater. No sanding is needed – the existing glaze surface provides enough texture for the fresh coat to grip. Fire to the same peak temperature with a 20-minute soak.
Why do pinholes appear more often on grogged clay bodies?
Quick Answer: Grog particles create pathways for gas to escape through the clay body wall during firing. Each grog particle leaves a micro-gap where it meets the finer clay particles, and gas follows these gaps to the surface. If the glaze has already begun to seal, the gas pushes through and leaves a pinhole.
Heavily grogged sculpture clays with 20-30% grog content are the most prone to this effect. The fix is extending the bisque hold at 1,650°F to 60 minutes, which allows gas to escape before the clay surface begins to seal.
For functional ware on grogged stoneware, using a slightly thinner glaze application helps the gas break through the melt layer more cleanly – the resulting pinhole is smaller and more likely to heal during the peak soak.
Conclusion
Pinholes in ceramic glaze are always caused by escaping gas – the only question is where the gas came from. Diagnose the source using the pattern of pinholes on your pot, then apply the specific fix: longer bisque burnout, correct glaze application thickness, a 20-minute peak soak, or a glaze chemistry adjustment.
Start with the free fixes first. Extend your bisque hold and add a peak soak before changing materials or recipes. For most potters, those two schedule changes eliminate pinholes entirely within one or two firings.
