Published Feb 12, 2024
In ceramics, one person's fault is another person's fancy. While crazing is technically a glaze fault, it can also be a very beautiful effect, and many potters seek to increase rather than reduce crazing.
Fortunately, this post can apply to both fancies because learning how to reduce crazing will also give you the knowledge on how to increase crazing! In this post, an excerpt from her book Special Effect Glazes, Linda Bloomfield tells you all you need to know to increase or reduce crazing in a glaze! - Jennifer Poellot Harnetty, editor
We will learn about crackle by first considering the options available for eliminating crazing. While gaining an understanding of how to either reduce or increase crazing, at the same time we will learn how to make crackle glazes.
Adding Silica and Clay to Reduce Crazing
There are several ways to correct crazing. However, changing only one material may change the appearance of the glaze, making it more glossy or matte. A reliable method is to increase both the silica (flint or quartz) and clay in the ratio 1.25:1 silica to clay.
Data from R.T. Stull. 1912. Graphic by Henry Bloomfield. Thanks to Matt Katz of Ceramic Materials Workshop.
Stull map of alumina and silica in porcelain glazes fired to cone 11 with constant flux 0.3 K2O and 0.7 CaO. By increasing the alumina and
silica in the molecular ratio 1:8, you can move a glaze from the crazed area (b) to the glossy, uncrazed area (a) and vice versa. Glossy, crazed glazes are found in the bottom left-hand corner, while Snowflake crackle is on the semi-matt 1:5 line
(see Kuan glaze opposite). The ratio of 1:5 alumina to silica gives a semi-matt glaze, while 1:8 gives a shiny glaze. The straight lines on the chart represent alumina:silica ratios of 1:4 (matt), 1:5 (semi-matt) and 1:12 (shiny, crazed glaze). The
dashed line is 1:8 Al2O3:SiO2 (bright, shiny glaze). Data from R.T. Stull. 1912. Graphic by Henry Bloomfield. Thanks to Matt Katz of Ceramic Materials Workshop.
This ratio comes from a series of porcelain glaze tests made by R.T. Stull in the USA in 1912. He measured the effect on the glaze of changing the alumina and silica content. He found that a molecular ratio of 1:5 alumina to silica gives a matte glaze, while 1:8 gives a shiny glaze. Alumina is found in clay, which is added to glaze to stiffen it in the melt and prevent it running off the pot. Clay contains silica as well as alumina, and the ratio 1:8 alumina to silica molecules converts to a ratio by weight of 1:1.25 clay to silica.
To change your glaze recipe, add increments of 1% clay and 1.25% silica until the crazing disappears. This has the effect of moving along the dashed blue line in the graph and crossing the red line from the crazed to the glossy, uncrazed area. You will need to multiply the percentage by the total weight; for example, in a 100g dry batch of glaze, add 1g china clay and 1.25g quartz, then wet-sieve and test on a pot to see if the crazing is reduced. If not, add increments of 1g clay and 1.25g quartz, up to 4g clay and 5g silica and test again. The cracks should move further apart the more clay and silica are added. Test tiles are often too small an area to see crazing, so you may need to test on a larger bowl or plate. Crazing is also affected by the glaze thickness. A thinner application of glaze or the addition of more water is sometimes enough to reduce the crazing, but obviously for a special effect we may want it to craze, so to increase crazing, reduce the silica and clay in the glaze and apply the glaze more thickly
Adding Fluxes to Reduce Crazing
Adding too much silica and clay may make the glaze appear underfired, dry and matte. Another way to correct crazing is to add a low-expansion flux material such as talc, which is magnesium silicate. Both magnesium oxide and silica have low expansion; both will decrease the expansion and contraction of the glaze during cooling, to help prevent crazing. Talc is a better choice than dolomite, which contains both magnesium and calcium oxide, the latter of which has a relatively high expansion (although lower than that of sodium or potassium). If you look at the fluxes on the next page, listed in order of decreasing expansion, with high expansion at the top and low expansion at the bottom, you can choose the most appropriate low-expansion flux to add to your glaze. It is simpler to increase a material already in the glaze than to add a new one. Stoneware glazes can have the addition of 5% talc or zinc oxide. However, these fluxes may affect the color of chrome green and chrome-tin pink glazes. Earthenware glazes can have the addition of a flux containing boron, which has a very low expansion and can be added in the form of calcium borate frit (or colemanite). Opacifiers and coloring oxides such as zirconium silicate and tin oxide also help to reduce crazing.
- Sodium oxide (in nepheline syenite and high-alkaline frit)
- Potassium oxide (in potash feldspar)
- Calcium oxide, strontium oxide
- Barium oxide
- Titanium oxide, lead oxide
- Lithium oxide (in lithium carbonate and spodumene)
- Zinc oxide
- Magnesium oxide (in talc)
- Tin oxide, zirconium oxide
- Alumina (in clay)
- Boron oxide (in calcium borate frit)
However, adding a flux will often make the glaze more runny. Alternatively, you can remake the glaze, reducing the high-expansion materials such as feldspar, or substituting low-expansion materials for the high-expansion ones, such as a lithium feldspar for soda feldspar or calcium borate frit for high-alkaline frit. It is a good idea to change one material at a time to keep a clear idea of the effect of each individual material on the glaze. On the other hand, if you want to induce crazing, add a high expansion flux such as nepheline syenite or a high-expansion frit such as high-alkaline frit (or Ferro frit 3110). Increasing the fluxes has the same effect as reducing the clay and silica in the glaze. Glazes consisting of 50% or more feldspar or nepheline syenite are likely to craze. Snowflake crackle (kuan) glazes are semi-matte glazes consisting mostly of nepheline syenite and are applied very thickly. On the Stull map, these Snowflake crackle glazes lie on the 1:5 semi-matte line, while glossy crackle glazes low in silica and alumina lie in the crazed area in the bottom left-hand corner.
Biaxial grid with high-alkaline glaze and copper oxide brushed on porcelain. Silica increases from 0-50% left to right. Clay increases from 0–40% bottom to top. The high-alkaline fluxes (Na2O+K2O+Li2O 0.7, CaO 0.3) remain constant throughout. The only uncrazed tile is in the centre row, fourth from left (14 kaolin, 34 silica). Interestingly, the copper changes from turquoise to green as more clay is added, which is why transparent turquoise glazes are often crazed. Grid made by May Luk and the London Potters glaze group.
Changing the Clay Body or Firing Temperature
If you do want crazing to happen, try lowering the firing temperature. Avoid adding extra silica or sand to the clay body. Conversely, ways to correct crazing include changing to a different clay body which better fits the glaze, adding silica to the existing clay body or increasing the firing temperature. Commercial clay bodies often already have silica added to prevent crazing. In stoneware, the addition of silica sand to the clay body can help prevent crazing. In earthenware, bisque firing to a higher temperature can eliminate crazing.
To increase crazing:
- Reduce silica and clay in the ratio 1:1.25 clay to silica by 5% silica and 4% clay
- Add nepheline syenite or lithium carbonate
- Substitute high-alkaline frit for borate frit
- Apply glaze very thickly
- Reduce firing temperature
To reduce crazing:
- Increase silica and clay by 5% silica and 4% clay
- Add 5% talc or zinc oxide
- Substitute lithium feldspar for sodium feldspar
- Substitute borate frit for high-alkaline frit
- Apply glaze thinly
- Increase firing temperature