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Steentjes in glazuur

Help, little stones in my glaze!

Have you ever experienced this? You give your favourite dipping glaze a good stir before using it and suddenly notice: “Help, there are little stones in my glaze!” Strange, because they certainly weren’t there when you first started using the glaze. What are they? Where do they come from? And, most importantly, how do you solve this problem?

No stones but crystals

In some glazes, small stones can appear over time. They are not actually stones but crystallised glaze ingredients. They can range in size from just a few millimetres to several centimetres. Some are smooth and rounded, while others are rough and angular.

Water solubility

The crystals are caused by glaze ingredients that are partially soluble in water. These are the flux oxides, such as lithium oxide, sodium oxide and calcium oxide. They originate from raw materials such as:

  • nepheline syenite
  • some frits
  • sodium carbonate
  • lithium carbonate

Using these raw materials does not automatically mean that crystals will form in your glaze suspension. There are likely other raw materials as well that can cause crystallisation.

The conditions, both chemical and physical, need to be just right. In my experience, it mainly happens after a cold period in the studio and only with certain glazes. A glaze that is used frequently is less likely to develop crystals than one that sits unused for a long time while being exposed to fluctuating temperatures.

Uitgekristalliseerde natrium in een glazuur

Are those crystals problematic?

It may seem a little annoying, but are these crystals really a problem?

The answer is simple: yes!

  • The crystals remain as hard lumps attached to the glazed pots and melt very differently from the rest of the glaze. This can create unusual, though sometimes quite interesting, colours and patterns in the fired glaze.
  • The composition of the glaze in the bucket is no longer the same. That makes sense, as the crystals consist of some of the flux oxides. As a result, fewer of these oxides remain available in the glaze itself. The glaze will therefore behave differently in the kiln and will likely melt less effectively.

How do you solve this?

How do you solve the problem? Simple. You have to dissolve it, quite literally. The crystals consist of water-soluble raw materials, so they can simply be dissolved again in a small amount of water. Once dissolved, add the solution back to the glaze suspension and stir thoroughly.

Deels opgeloste natrium kristallen

Can't I just throw them out?

You should never simply throw the crystals away. If you do, you are discarding important glaze ingredients and altering the recipe. The glaze will therefore behave differently and may no longer mature properly. The colour and even the texture may change as a result.

Practical approach

  1. Sieve the glaze with a glaze sieve (60-100 mesh)
  2. Scrape as many crystals as possible from the sieve.
  3. Place them all in a glass or a microwave-safe container.
  4. Voeg een heel klein beetje water toe zodat ze net onder staan
  5. Place the container with the crystals in the microwave for a few minutes.
  6. Repeat step 5 if not all crystals have dissolved. If necessary, add a little more water.
  7. Add the liquid containing the dissolved crystals back into the glaze and stir thoroughly.

Repeat these steps if crystals form again after some time.

If you prefer not to heat glaze materials in a microwave, you can also add hot water to the crystals. It will require a lot more patience, but with time and regular stirring, the crystals will eventually dissolve.

Still some questions?

Do you still have questions after reading this article? Whether it's about this topic or any other clay, glaze, or kiln issue you're struggling with, you can book an online consultation with me. Sessions are available in 15, 30, or 60-minute formats. You're very welcome.