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2019 Jiggering-Casting Project
Beer Bottle Master Mold Project
Build a kiln monitoring device
Comparing the Melt Fluidity of 16 Frits
Cookie Cutting clay with 3D printed cutters
Evaluating a clay's suitability for use in pottery

Make a mold for 4-gallon stackable calciners
Make Your Own Pyrometric Cones
Making a high quality ceramic tile
Making a jigger mold for producing cereal bowls
Making a Plaster Table
Making Bricks
Making our own kilns posts using a hand extruder
Making your own sieve shaker
Medalta Jug Master Mold Development
Mother Nature's Porcelain - Plainsman 3B
Nursery Plant Pot
Pie-Crust Mug-Making Method
Plainsman 3D, Mother Nature's Porcelain/Stoneware
Project to Document a Shimpo Jiggering Attachment
Roll, Cut, Pull, Attach Handle-making Method
Slurry Mixing and Dewatering Your Own Clay Body
Testing a New Load of EP Kaolin
Using milk as a glaze

G3948A Iron Red glaze: Can you help?

The G3948A iron red glaze recipe was working so well. Its chemistry was within limits to make this type of glaze. Initial results were so promising and many successful pieces were made with both the C6DHSC and C6IRED firing schedules on many types of clay bodies. But when remixed it is completely different. Some people are making it with success and others are reporting the same problem I am having. A lot of people really want to use this so if you have any ideas on what is wrong could you please contact me?

Related Information

Original G3948A iron red cone 6 oxidation glaze

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This is recipe G3948A fired using our standard C6DHSC schedule. This was the first specimen out of the kiln that got me really excited.

New iron-red glaze on porcelain at cone 6 oxidation

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Two iron red mugs

This is the G3948A recipe. Iron red glazes are easy to do in high-temperature reduction but not so in medium-temperature oxidation. Most people just try a bunch of recipes they find online hoping that one of them actually fires the way it is shown in the picture! A better approach for us was to study a range of ones claiming to be iron reds looking for things in common with the chemistries and recipes. G3948A, on these two M370 mugs, is a product of that. Unlike many, the original recipe we found, G3948, did have a suggested firing schedule. It seemed strange so we just used the standard C6DHSC slow-cool schedule. That one is also ideal for the liner glazes, giving them a better gloss finish. It was not tempting to even try the original recipe (because it measured up poorly against common sense recipe limits), but it did make sense to fix obvious issues and then try it. Unlike every other recipe we have seen, this one suffers no issues with gelling of the slurry because it contains no Gerstley Borate and uses black iron oxide. It has very good application properties and requires only 80 water for each 100 powder to mix as a creamy dipping glaze. And it does not need any lithium carbonate.

Same recipe, same clay, same firing schedule. What went wrong?

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The glaze thickness is also the same. The firing schedule is critical with iron reds, we tried both the C6DHSC and C6IRED schedules, both of which normally produce the result on the left (with this G3948A recipe). But in both firings the result is now like the piece on the right. The one difference is this: The new batch was ball milled. My first theory was this: Iron oxide particles agglomerate and neither our propeller mixer or blender remove these agglomerates - but the ball mill does. The agglomerates must be either seeding the crystals or facilitating their growth. However subsequent failures in mixing the recipe without ball milling did not solve the problem. But neither did they melt as well as this milled version.

Same recipe, same preparation, same clay, same firing schedule

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G3948A has stopped working. It is early 2023 and we have mixed it again multiple times and each time it is firing this ugly brown! I have each original mix ticket and have triple-checked each. I have tested both frits using a GBMF test and they are not mislabelled. The other materials are from the same 50lb bags. In the end the problem turned out to be restocking of our TriCalcium Phosphate container with Pyrophyllite. When that ran out completely and the empty jar was restocked the glaze suddenly began working again!

Why does fluid-melt G3948B not produce an iron red

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Stull chart for iron red glazes

Chazo Chazim Mehmeti made this Stull chart to help explain why my G3948B does not produce iron crystals. It plots the formula amounts of Al2O3 (vertical axis) vs. SiO2 providing one lens through which to view the chemistry of multiple iron red glaze recipe candidates that work and don't work. His argument is that, among other necessary things like the presence of P2O5, MgO and CaO, the amounts of SiO2:Al2O3 must be within certain bounds (this chart conveys both the ratio and amounts). He points out that the relationship is sensitive enough that one should use the closest possible chemical analysis of the materials (not generic ones) when plotting points on the chart. Over a year of testing with his students, the ones fitting in the small zone on the chart worked every time (#2 being best). That is where my G3948A, which is working really well, also resides. The G3948B, which does not work, appears way off to the right because of its high SiO2 content. Of course, there could be other reasons for its failure, but the SiO2 issue is a good place to start.

By Tony Hansen
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