Why did my glaze do that?
This is a practical guide that lists some of the most common reasons for different glaze defects.
BEFORE YOU BLAME THE GLAZE...
Glaze can be a little mysterious.
You put what looks like a perfectly innocent layer of glaze onto your pot, put it in the kiln, or drop it off at a firing service and sometimes it comes out exactly as planned. And sometimes it crawls, cracks, bubbles, runs, goes weirdly matte, changes colour or appears to have developed a personal grudge against you. The good news? Glaze defects usually aren't random. The trick is learning how to read the evidence and figure out what went wrong.
Before you change your glaze recipe, your firing schedule, your clay body, before you blame your kiln or mercury in retrograde and everything else in sight, take a breath and investigate.
Start with these questions:
What exactly happened?
Is it happening on every piece or just some?
Is it happening with one glaze or several?
Is it happening on one clay body or several?
Was the glaze applied the same way?
Was the bisque fired normally?
Has the glaze batch changed?
Does the problem appear before firing, during firing, or after cooling?
The goal isn't to find someone to blame. It's to find the variable that changed.
THE GOLDEN RULE
Change ONE thing at a time.
This is possibly the most useful piece of glaze troubleshooting advice we can give you.
If you change:
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the glaze
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the clay
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the application
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AND the firing
all at once...you have no idea which change fixed the problem. Instead, make a controlled test.
TEST A Your normal process.
TEST B Change one variable.
TEST C Try another controlled variation.
Then compare the results.
Think like a scientist.
Experiment like a potter.
Glaze Troubleshooting: The Big Four
A quick way to diagnose most glaze surprises
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Clay |
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Application |
Why did my glaze do THAT? |
Glaze |
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Firing |
Tip: When troubleshooting, change one of these variables at a time so you can see what actually made the difference.
1. CRAWLING
What does it look like?
The glaze pulls away from the clay during firing, leaving areas of bare clay surrounded by raised or beaded edges. It can look like the glaze has shrunk away from itself.
FIRST THINGS TO CHECK
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Is the glaze too thick? A thick glaze layer is the most common cause for crawling and one of the first things to investigate.
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Was the bisque clean? Dust, grease, fingerprints and other contamination can interfere with the glaze's adhesion.
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Is the glaze cracking while it dries? A glaze layer with high dry shrinkage can crack during drying and then crawl during firing.
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Is the surface unusually smooth or contaminated? Some surfaces make it easier for a glaze to pull together rather than remain evenly attached.
TRY THIS
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→ Clean your bisque thoroughly.
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→ Apply a thinner coat.
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→ Allow each coat to dry properly.
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→ Look closely at the glaze before firing. Is it already cracked?
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→ Test the glaze on a clean tile.
IF IT KEEPS HAPPENING
Start investigating the glaze composition, particularly the amount and type of clay or other materials contributing to dry shrinkage. Don't immediately assume the glaze is faulty. Application and surface preparation are often the first places to look.
2. PINHOLES
What do they look like?
Tiny holes or pinpricks that break through the glaze surface.
WHAT'S HAPPENING?
During firing, gases are released from the clay and/or glaze. As the glaze melts, those gases travel through the molten glaze. If a bubble reaches the surface but the glaze doesn't have enough time or fluidity to heal the hole, a pinhole can remain.
THINGS TO INVESTIGATE
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Bisque firing - Was it adequately fired?
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Glaze thickness - Is the glaze unusually thick?
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Glaze fluidity - Is the molten glaze able to heal the surface after bubbles escape?
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Clay body - Could the clay be contributing significant gas during firing?
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Surface - Are pits, grog or texture creating places where bubbles can become trapped?
TRY THIS
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→ Check your bisque firing schedule.
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→ Apply the glaze evenly.
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→ Test a thinner application.
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→ Test on a different clay.
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→ Test a controlled 10 minute hold at peak temperature.
3. BLISTERS
What do they look like?
Larger bubbles, raised areas or craters in the glaze surface. Like tiny evil moonscapes.
WHAT'S HAPPENING?
Off gassing, gas bubbles become trapped in the molten glaze. If they cannot escape, or the glaze can't heal the surface after they do, they can leave bubbles or craters behind.
CHECK:
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Is the glaze too thick?
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Is the glaze being over fired?
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Is the glaze particularly viscous? (thick like honey)
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Is the clay producing significant gas?
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Has the firing schedule changed?
TRY:
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Thinner application
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Check heat work
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Test firing schedule
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Test another clay body
More heat isn't automatically the answer. In some situations, additional heat work can make blistering worse.
4. CRAZING
What does it look like?
A network of fine cracks running through the glaze surface. Sometimes you may hear the glaze pinging as the piece cools, but sometimes crazing is delayed and it can happen weeks or even months later.
WHAT'S HAPPENING?
The glaze and clay body expand and contract at different rates. The glaze is like a tight pair of jeans stretching over a bulky clay body! If the glaze contracts more than the clay as the piece cools and the glaze can be placed under tension and crack. This is called glaze fit.
THE IMPORTANT BIT
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A glaze doesn't exist in isolation.
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A glaze that works beautifully on one clay body may craze on another.
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So ask: Does it happen on one clay body or several?
*We see this quite often in the shop when a clay body is used with a glaze that doesn’t match it’s firing temperature. e.g A stoneware clay with a midfire glaze or a stoneware clay being under fired at midfire. Crazing happens more often in Earthenware.
FIRST TEST
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Check the recommended glaze and clay temperatures. Some commercial clay and glazes suggest a very wide firing range, but they are likely to be under-fired at the bottom temperature or over fired at the higher temperature, or both.
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Try the glaze on another compatible clay body.
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Try firing it lower or higher (according to its recommended range.)
If the crazing disappears, you've learned something important: The problem may be the glaze + clay combination rather than the glaze alone.
FOR FUNCTIONAL WARE
Crazing should be taken seriously. A crazed glaze may be less durable and can create additional concerns for functional surfaces, creating the opportunity for bacteria or even mould to grow.
DON'T JUST GUESS
Glaze fit is a chemistry problem. If you're changing a glaze recipe to address crazing, make small, controlled adjustments and test them. Glaze fit is a relationship, not a personality trait.
5. SHIVERING
What does it look like?
The glaze flakes, chips or lifts away from the clay. It's often most obvious around:
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rims
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sharp edges
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handles
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corners
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changes in form
WHAT'S HAPPENING?
Shivering is essentially the opposite glaze-fit problem to crazing. The jeans are too big! The glaze is under excessive compression relative to the clay body and can release by lifting or flaking away.
IMPORTANT
Be carful handling pieces that have shivered. Do not use shivering ware for food or drink - its sharpy and glassy and pointy!
INVESTIGATE:
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Clay body (match the right clay and glaze)
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Glaze chemistry
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Slip, underglaze or engobe underneath - sometimes they create a fine layer of dust that prevents the glaze from adhering. Brush the glaze on well to clean, dry bisque ware
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Firing range (check it was fired to the correct temperature)
6. RUNNING & DRIPPING
What does it look like?
The glaze flows down the pot during firing. It ruins your pot and your kiln shelves! You may see:
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heavy pooling
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thin glaze at the top
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glaze running onto the shelf
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pieces stuck to kiln furniture
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A very distressed kiln technician sobbing in the corner (don’t be alarmed, bring her coffee and a grinder)
FIRST CHECK:
THICKNESS
A glaze that behaves perfectly at one thickness can become a completely different animal when applied heavily. Dripping is almost always from a heavy application. Take notes of your layers and thickness of application.
ALSO CHECK:
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Firing temperature / heat work - over fired glaze will also run, make sure you are firing to the correct temperature. Again - some commercial glazes list a wide firing range and the glaze may be over fired at the top end of this. For example Amaco and Mayco glazes list firing from cone 6-10 but will be over fired in a cone 10 and may run.
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Our River Rock glazes are best at cone 6-8, at cone 10 the glossy glazes will be more fluid.
TRY:
→ Apply less glaze. Reduce the number of coats.
→ Keep glaze away from the foot.
→ Use a catch tile/cookie when testing an unfamiliar glaze.
→ Check your firing is reaching the intended cone.
*Never put an unfamiliar glaze straight onto your favourite pot. Test it first.
7. UNEVEN OR PATCHY GLAZE
What does it look like?
You may see:
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thin areas
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brush marks
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uneven colour
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patches
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areas that look completely different from one another
CHECK YOUR APPLICATION
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How many coats did you apply?
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Was the glaze mixed thoroughly?
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Was the glaze consistent from beginning to end?
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Did each coat dry before the next?
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Was the bisque absorption even?
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Was the glaze allowed to settle?
REMEMBER
Two pieces can be dipped in the same glaze and still receive very different amounts of glaze. Clay type will determine the glaze thickness because some clay bodies will vitrify more at bisque and become less porous. BRT for example.
TRY THIS
Make a test tile with:
1 coat | 2 coats | 3 coats | 4 coats
Fire it. Now you have evidence rather than guesswork.
8. "THE COLOUR IS DIFFERENT!"
This is one of the most frustrating kiln surprises. You used the same glaze...but it doesn't look like your test tile.
CHECK THESE FIRST:
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CLAY: Is it the same clay body?
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APPLICATION: Is the thickness the same?
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FIRING: Was the heat work the same? Was it over or under fired?
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GLAZE: Was it mixed every time? is the specific gravity the same? Is it the same batch, thickness and consistency?
ALSO CONSIDER:
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texture
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layering
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number of coats
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firing schedule
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atmosphere
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cooling
A glaze isn't a paint colour. It's a ceramic material undergoing a chemical and physical transformation in the kiln. That's why test tiles are so valuable.
*In the shop we notice the most common cause of ‘colour change shock’ is uneven application (often too thin).
"BUT IT WORKED LAST TIME!"
The classic pottery question.
SAME GLAZE. SAME CLAY. SAME KILN. COMPLETELY DIFFERENT RESULT.
Before you blame the glaze, compare the five things that may have changed:
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CLAY: Same clay? Same batch?
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GLAZE: Same batch? Same consistency?
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APPLICATION: Same thickness? Same number of coats?
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FIRING: Same schedule? Same heatwork?
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KILN: Same kiln? Same position?
BECOME A GLAZE DETECTIVE
When something goes wrong, don't just ask: "How do I fix it?" Ask: "What evidence do I have?" Write down:
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WHAT I SEE
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WHERE IT HAPPENED
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WHAT CLAY I USED
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WHAT GLAZE I USED
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HOW I APPLIED IT
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HOW IT WAS FIRED
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WHAT WAS DIFFERENT THIS TIME?
QUICK TROUBLESHOOTING GUIDE
PROBLEM |
START HERE |
Crawling |
Check thickness, bisque cleanliness and dry cracking |
Pinholes |
Check clay, bisque, thickness and firing |
Blisters |
Check thickness, heatwork and glaze fluidity |
Crazing |
Investigate glaze + clay fit |
Shivering |
Investigate glaze + clay fit immediately |
Running |
Check thickness and heatwork |
Patchy coverage |
Check application, mixing and absorption |
Lumpy glaze |
Check mixing, settling and consistency before adding water |
Unexpected matte surface |
Check thickness, heatwork and whether the surface is actually a defect |
Unexpected colour |
Compare clay, glaze, application, firing and kiln position |
Make tests!
Every kiln firing teaches us something. Test tiles are fantastic resources so you make informed decisions about glaze and decoration! and they look great! make fun test tiles and think of interesting ways to display them. Try:
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different thicknesses
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different textures
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different clay bodies
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different glaze combinations
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different firing conditions
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Then keep the ones that teach you something.
Because the best glaze results aren't always the ones you planned. Sometimes they're the ones you discover.
WANT TO GO FURTHER?
KEEP EXPERIMENTING.
River Rock Glazes was developed in the Northern Rivers by potters who love reliability but embrace the slightly unpredictable, wonderfully chemical world of ceramic glaze.
We make our glazes for Australian potters with beautiful surfaces, reliable results and plenty of room to experiment.
Explore the River Rock range
27 mid-fire colours
Gloss • Matte • Speckled
Show us what you discover!
Tag @riverrockglazes
We love seeing what happens when potters take a glaze somewhere we didn't expect.
@riverrockglazes
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