Common sense approach to finding a Raku crackle recipe

Project Name

Raku Crackle Glaze Recipe

Project Codenumber

UnAssigned

Notes

Making a raku crackle glaze is better done using common sense than trying a bunch of found-recipes on line.

The first thing common sense thing you can to do is eliminate recipes that are obviously going to be troublesome or won't work. The first column is the one I think you should use, I have notes on the others about what their issues are.

Notice the thermal expansion is calculated for each of these. If they are going to craze then that expansion needs to be high as possible. Of course, that weakens the pieces a lot, but we are assuming you only care what they look like anyway. The universal method to raise thermal expansion at all temperatures is to maximize the sodium (Na2O). Nepheline syenite and soda feldspar are the best raw sodium-contributors at middle and high temperatures. But Ferro Frit 3110 is far better at lower temperatures, since it melts so well.

All the traditional recipes we found online have an extremely high percentage of Gerstley Borate (GB). GB is the source of boron, that is what makes them melt at a low temperature. Given the ease of getting boron frits it is a little crazy to use GB (it is often referred to as "Ghastley Borate" for good reason): All of these are going to be buckets of pure jelly (GB gels slurries). To make them liquid enough much more water will be needed, that will waterlog pieces as you glaze them.

It makes better sense to use a frit that melts low and has a high thermal expansion. That is, again, Ferro Frit 3110. And it is easy to get. The simplest recipe could be 100% frit 3110. But adding 15% kaolin will produce an easy-to-use slurry that will suspend and harden on the ware.

Add 10-20 Zircopax if you want white.

If you know anything about glaze chemistry, it is interesting to compare the oxide formula's of all of these.

Raku Crackle Glaze Base - Frit 3110

Total:100.00

Auto Unity Formula

Notes

*This recipe makes the most sense. It just suspends the Ferro Frit 3110 with 15% kaolin. Frit 3110, all by itself, is a perfect raku glaze, very high thermal expansion and it melts well to a crystal clear glass at low temperatures. Why use anything else?

This recipe is displayed on the low temperature Plainsman clay body sample boards (for Raku clay). These crazed badly immediately on removal from the test kiln.

Having this much frit this glaze will want to mix to a very high specific gravity yet still be watery (one user called it “skim milk thin” even with 70:100 water:powder and 1.6 specific gravity). There are a few things you can do about this. First, adjust it to be thixotropic that will enable reducing the specific gravity and improving the application properties. Another option is to use ball clay instead of kaolin, it has almost the same chemistry (it just has more iron and will stain the clear glass slightly amber). The third option is to convert it to a brushing glaze. Although most potters balk at this idea it is actually practical - with a good brush and some practice you can control thickness (simply by the number of coats applied) - it is even possible do very large pieces.

Add 10-20 zircopax to make it white.

Pictures

Untitled

Raku Glaze Base #1

Total:100.00

Auto Unity Formula

Notes

*A big issue with this recipe is getting Frit 5301. Even if you can get it, do you want to use a material that generates fluorine gas on firing? And mix it with a material that will turn the slurry into a jelly?

Another issue with this recipe is that it is often opacified with 10% or more zircopax. But in some place this recipe abbreviates that as "zirc", that in turn has morphed to "zinc" (which people interpret as zinc oxide). Of course that is not going to produce a white glaze. And the zinc will reduce expansion, therefore reducing crazing.

Bottom line: Leave this one on the page where you found it.

Pictures

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Raku Glaze Base #1" id="205742" key="petXjdtS" date="2021-09-07" codenum="L4264A"> <recipelines> <recipeline material="Ferro Frit 5301" amount="40.000" tolerance=""/> <recipeline material="Gerstley Borate" amount="60.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2020-04-01, Modified: 2021-09-07 17:26:37

Raku Glaze Base #2

Total:100.00

Auto Unity Formula

Notes

*This also relies on Nepheline Syenite to source the Na2O needed to increase the thermal expansion (to cause the crazing). But 20% may not be enough to do that. And, 80% Gerstley Borate, the highest percentage of any recipe we found, is going to require so much water to create a slurry, that the glaze will crack badly as it is drying on the piece. Then pieces will waterlog, preventing getting the layer thick enough.

Another issue is the LOI of GB. The gases produced as GB decomposes during melting produce bubbles and bubble-clouds in the glass.

Unless you are a masochist, leave this one alone.

Pictures

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Raku Glaze Base #2" id="205743" key="LNopyDvp" date="2021-09-07" codenum="L4264B"> <recipelines> <recipeline material="Nepheline Syenite" amount="20.000" tolerance=""/> <recipeline material="Gerstley Borate" amount="80.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2020-04-01, Modified: 2021-09-07 17:30:03

Raku Glaze Base #3

Total:100.00

Auto Unity Formula

Notes

*This recipe has a big problem: Spodumene is a low expansion material, to get a glaze to craze a high-expansion material must be added to the Gerstley Borate.

Another issue: The ball clay is not needed, GB is a clay and it will have no trouble suspending the Spodumene.

Another problem: Spodumene creates a very annoying dust, likely not healthy. Even tiny amounts are a big nuisance. And it does not wet well, slurries containing spodumene are full of bubbles that are almost impossible to get rid of, that makes it impossible to get an even layer on the ware.

Pictures

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Raku Glaze Base #3" id="205744" key="wWFgHSZi" date="2021-09-07" codenum="L4264C"> <recipelines> <recipeline material="Spodumene" amount="35.000" tolerance=""/> <recipeline material="Gerstley Borate" amount="60.000" tolerance=""/> <recipeline material="Ball Clay" amount="5.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2020-04-01, Modified: 2021-09-07 17:33:18

Raku Glaze Base #4

Total:90.00

Auto Unity Formula

Notes

*An issue with this base: It is not likely to craze enough. While the 15 Nepheline Syenite should cause some crazing, the ball clay and silica additions reduce the thermal expansion. Ball clay is good to have for suspension but there is plenty of Gerstley Borate to do that, so maybe kaolin would be better. One improvement would be to remove the silica and replace it with nepheline. If it still does not craze enough replace the kaolin with Nepheline also.

This recipe is also being used with Gillespie Borate. It should not gel the glaze as much.

Pictures

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Raku Glaze Base #4" id="205745" key="iAdZ3nZt" date="2024-05-20" codenum="L4264D"> <recipelines> <recipeline material="Nepheline Syenite" amount="15.000" tolerance=""/> <recipeline material="Gerstley Borate" amount="65.000" tolerance=""/> <recipeline material="Ball Clay" amount="5.000" tolerance=""/> <recipeline material="Silica" amount="5.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2020-04-01, Modified: 2024-05-20 16:00:23