BWIW - Boiling Water:Ice Water Glaze Fit Test

This test subjects the clay:glaze interface to a differential thermal stress of 180 degrees F (100C) both hot-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 to functional ware strength and safety.

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 can be employed to adjust glaze expansion while maintaining other fired properties.

Cautions:

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.

Low expansion glazes craze less, but they can shiver

Example of serious glaze shivering using G1215U low expansion glaze on a high silica body at cone 6. Be careful to do a thermal stress test before using a transparent glaze on functional ware.

Why are these crazing lines dark like this?

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.

A Redart cone 03 body shines when it come to ease of glaze fit

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.

Turning delayed crazing into immediate crazing

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. This is the 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 high expansion Frit 3110.

Variables

Val - Value (V)

Procedure

100. Apparatus

-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)
-Timer
-Dipping tongs

200. Procedure

-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 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.

Since time is also a big factor in crazing and shivering, this test can be time-extended. Put items in a freezer for a few days after a more extended time in boiling water (e.g. an hour).

Related Information

Links

Glossary	Glaze shivering 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.
Glossary	Co-efficient of Thermal Expansion The co-efficient of thermal expansion of ceramic bodies and glazes determines how well they fit each other and their ability to survive sudden heating and cooling without cracking.
Glossary	Food Safe 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.
Media	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).
Articles	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.
Articles	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.
Tests	Co-efficient of Linear Expansion In ceramics, glazes expand with increasing temperature. Being brittle materials, they must be expansion-compatible with the body they are on.
Tests	Glaze Melt Flow - Runway Test A method of comparing the melt fluidity of two ceramic materials or glazes by racing them down an inclined runway.
Tests	300F:Ice Water Crazing Test Ceramic glazes that do not fit the body often do not craze until later. This progressively stresses the fit until failure point, thus giving it a score
Typecodes	Glaze Tests Tests conducted on glaze batches used in production (as opposed to tests conducted on the materials used to make those glazes).
URLs	https://www.eieinstruments.com/tiles_&_ceramics_testing_instruments/autoclave_test/tiles-autoclave-crazing-test-autoclave-for-website Tiles Autoclave - Crazing Test Autoclave
Troubles	Glaze Crazing Ask the right questions to analyse the real cause of glaze crazing. Do not just treat the symptoms, the real cause is thermal expansion mismatch with the body.

By Tony Hansen
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