What is wrong with my pottery glaze?

Is your glaze acting up? There are a whole load of reasons that your glaze might not be working as you'd expect. Let's try to troubleshoot a few of them!

In general, most glaze issues stem from the following issues:

  • The recipe itself 
  • Glaze/clay incompatibility 
  • Dust/dirt on the bisque
  • Glaze not mixed properly
  • Application method 
  • Maturing temperature
  • Incompatibility of layered glazes or underglazes
  • Too much/too little water in the glaze

Let's look at specifics.

Issues that can occur before firing & when applying:

Flaking/Cracking when drying

Is it flaking/cracking when it dries after applying? Do you have a powdery bisque surface? This usually means that the glaze is too thick, or layered glazes did not dry enough before adding additional layers.

  • Potential solutions:
    • Use gum solution
    • Layer the glaze on top of another glaze that does not crack after applying
    • You may need to wash it off completely and try again

 

Is your glaze very powdery and delicate?

  • What is causing it?
    • Glaze has minimal clay content
  • Potential solution:
    • Reformulate the glaze with higher clay content or add 2% bentonite to the recipe.
    • If reformulation is not possible, mix a solution of equal parts water and liquid laundry starch, keep it in a spray bottle. Lightly spray this solution to effectively lock the glaze in place.

What if your glaze is old?

  • Add gum solution and equal parts distilled water. Add very little at a time. (ex: a teaspoon to tablespoon per half pint/full pint)

Glaze issues that show up during firing or after firing

Bald or thin patches?

Potential solutions:

    • Try dipping or pouring
    • Glaze is too thin or has settled; thicken it by leaving it out to evaporate a bit, and/or mix more often
    • You may have had an uneven application; try to make sure it goes on smoothly

Blisters:

Are you seeing large holes or craters in your glaze? Here are some causes and solutions for blistering:

  • Overfiring is causing glaze and/or clay to off-gas too much
    • Place items in a lower part of the kiln where it is cooler.
    • Fire at a lower cone
  • Glaze is too thick
    • Adjust glaze thickness
    • Apply less glaze
  • Firing issue: the glaze is caught in the middle of bubbling at mature temperature and hasn't been given time to heal holes
    • hold at mature temperature to allow for healing and extra off-gassing
  • Off-gassing without healing (usually dark colored clay bodies off-gas quite a bit)
    • Use a long, slow preheat to allow full off-gassing / use a slower firing schedule / hold at mature temperature
    • Or use a slower ramp when close to mature temperature

Pinholes:

If you're seeing small holes in the glaze, it may be caused by:

  • A dusty/dirty surface
    • Wipe the piece carefully before glazing
  • Glaze contamination
    • Sieve your glaze to remove contaminants like dust
  • Uneven application
    • Try dipping, or pouring if dipping isn't possible
  • A firing issue
    • hold at mature temperature to heal holes
  • The type of clay body allows lots of air pockets (for instance, it has sand, grog, or poor pugging)
    • choose a finer clay
    • add a layer of slip before glazing

Crawling:

Crawling is the pilling or contracting of the glaze into little balls, leaving bare areas. This is caused by poor bonding of the clay and glaze. A simple fix is to refire with the problem glaze layered on top of one that you know bonds fine with your clay. Here are some other specific issues and their solutions:

  • A dusty/dirty/oily bisque surface
    • Wipe or wash bisque and let dry before glazing
  • Surface is too smooth
    • Don't smooth the surface too much during creation
  • Glaze is too thick
    • Apply less, or adjust the thickness of the glaze itself
  • The glaze application method
    • Try a different method (dipping or pouring)
    • If you're applying the inside and outside separately, try to apply them as close to the same time as possible
    • If layering, try to apply as close as possible to the same time
  • Glaze has settled
    • Mix well (and often!) when applying
  • Bisque firing was hotter than normal
    • Try lowering the bisque firing temperature
  • Piece is too thin, and was dipped
    • Try creating slightly thicker walls and using a different application method
  • Force pulling the glaze away from the surface due to high shrinkage
    • Change the clay body 

Crazing:

Crazing is hairline cracks in the glaze that are caused by the glaze shrinking more than the clay. It looks similar to a crackle effect. This doesn't always show up immediately when you take the piece from the kiln, and can show up later. Contrary to popular opinion, opening your kiln when it's at room temperature after firing won't stop crazing, since it's more than likely caused by the relationship between the clay and the glaze. A general fix is to apply your glaze more thinly to cope with stress better, but here are some more specific causes and solutions to glaze crazing:

  • The clay is expanding more than the glaze:
    • Test different clay/glaze combinations for one that is more compatible
  • If low-fire, it may have absorbed water and expanded
    • Leave an area unglazed and plug it with a silicone sealant to prevent further water absorption from elsewhere

Shivering:

The opposite of crazing, shivering is when the glaze flakes off in patches form being compressed by the clay. This happens more often at low temperatures and with brown burning stoneware. It can also cause dunting, when the clay cracks instead of the glaze because it's in a position where it's not possible to shiver off (such as inside of a mug)

  • The clay expands and moves more than the glaze
    • Test for a more compatible combination
    • adjust your glaze recipe
    • Try a thinner application

Metal Marking:

Metal marketing is when cutlery leaves marks on the glaze when the piece is used. This is not that the glaze is weak; on the contrary, it means that metal is being scraped off the cutlery by a strong glaze. Here are some causes and solutions to try:

  • Glaze chemistry
    • try a mature gloss glaze rather than a matte glaze
  • Surface texture
    • If clay is coarse/groggy, try a smoother clay or surface treatment
    • Change your clay body

Do you still have questions? We highly recommend Digital Fire's glossary on ceramic glaze defects. They are a great resource for troubleshooting!