|Monthly Tech-Tip |
This test subjects the clay:glaze interface to a differential thermal stress of 180F (112C) both hold-to-cold and cold-to-hot (the former accelerates crazing and the latter shivering. This test is needed because, although ware may appear OK when first removed from the kiln, over time less-than-ideal fit will reveal itself. Compatibility between the expansions of clay and glaze are critical, not only to the integrity of the glaze layer, but also functional ware strength. Crazed glazes also provide a channel for water absorption by a porous body (creating conditions for the harboring of bacteria). Shivering glazes can drop chips of razor-sharp glaze flakes into food or drink.
While many people feel that dilatometer-measured thermal expansion numbers from body and glaze are needed to match a glaze to a body properly, the real truth is that no matter what the numbers say, the actual performance of the glaze-body system, when subjected to sudden temperature changes in real use, is a fool-proof indicator.
Because it tests both sudden cooling and heating, this test provides a direction in which to move the thermal expansion of an ill-fitted glaze. If shivering occurs, expansion needs to be increased, vice versa if it crazes. Glaze chemistry (via Digitalfire Insight or Insight-live.com) can be employed to adjust glaze expansion while maintaining other fired properties.
Some people have found that although a glaze may pass this test without crazing, it may still craze over time. It appears that this test may not adequately stress the hot-to-cold fit. The 300F:Ice Water Glaze Crazing Test may be more appropriate for testing crazing (but not shivering) since it stresses the ware up to 270F (compared to 180 for this one).
Your test specimens should have a wall thickness that approximates that of ware you will produce. If your samples are thin walled the clay matrix will contract quickly as well (when immersed in the cold water) and crazing may not appear.
This is a cone 04 clay (Plainsman Buffstone) with a transparent glaze (G1916Q which is 65% Frit 3195, 20% Frit 3110, 15% EPK). On coming out of the kiln, the glaze looked fine, crystal clear, no crazing. However when heated to 300F and then immersed into ice water this happens (IWCT test)! At lower temperatures, where bodies are porous, water immediately penetrates the cracks and begins to waterlog the body below. Fixing the problem was easy: Substitute the low expansion Frit 3249 for the Frit 3110.
These bowls are fired at cone 03. They are made from 80 Redart, 20 Ball clay. The glazes are (left to right) G1916J (Frit 3195 85, EPK 15), G191Q (Frit 3195 65, Frit 3110 20, EPK 15) and G1916T (Frit 3195 65, Frit 3249 20, EPK 15). The latter is the most transparent and brilliant, even though that frit has high MgO. The center one has a higher expansion (because of the Frit 3110) and the right one a lower expansion (because of the Frit 3249). Yet all of them survived a 300F to icewater IWCT test without crazing. This is a testament to the utility of Redart at low temperatures. A white body done at the same time crazed the left two.
This is an example of serious crazing in a glaze. The lines have gotten darker with use of the bowl! That means the color is organic, from food. This cannot be healthy.
-Boiler (sufficiently large to hold a sample of your water or a large shard and completely immerse it quickly)
-Ice and ice water container (large enough to quickly and completely immerse the item being tested)
-Prepare the ice water container with enough water to immerse the object(s) and with enough ice to bring the water to near freezing temperatures and hold it there for the duration of the test.
-Select samples (or shards) of your ware that are representative of the varying glaze thickness, contours, glaze wrap-arounds and a larger flatter glazed areas.
-Immerse the item(s) to be tested in the boiling water for three minutes.
-Move them to ice water for three minutes.
-Repeat three times.
-Use a dye, ink or a black marker (followed by cleaning with an appropriate solvent) to highlight crack lines.
Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.
Co-efficient of Thermal Expansion
Ceramics are brittle and many types will crack if subjected to sudden heating or cooling. Some do not. Why? Differences in their co-efficients of thermal expansion.
Shivering is a ceramic glaze defect that results in tiny flakes of glaze peeling off edges of ceramic ware. It happens because the thermal expansion of the body is too much higher than the glaze.
There is an increasing awareness of the food safety of glazes among potters. Be skeptical of claims of food safety from potters who cannot explain or demonstrate why.
|Tests||300F:Ice Water Crazing Test|
|Tests||Glaze Melt Flow - Runway Test|
|Tests||Co-efficient of Linear Expansion|
Tests conducted on glaze batches used in production (as opposed to tests conducted on the materials used to make those glazes).
Is Your Fired Ware Safe?
Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it could harbor bacteria and it could flake of in knife-edged pieces.
Crazing and Bacteria: Is There a Hazard?
A post to a discussion on the clayart group by Gavin Stairs regarding the food safety of crazed ware.
Desktop Insight 3 - Dealing With Crazing
Learn what crazing is, how it is related to glaze chemistry, how INSIGHT calculates thermal expansion and how to substitute high expansion oxides (e.g. Na2O, K2O) with lower expansion ones (e.g. MgO, Li2O, B2O3).