|Monthly Tech-Tip |
Modified: 2022-03-03 13:56:38
Plainsman Cone 6 Ravenscrag Slip based iron-red glaze. It can be found among others at http://ravenscrag.com.
|Ferro Frit 3134||14.10||16.6|
|Red Iron Oxide||15.00||17.65|
Iron red glazes are common, but tricky, in the cone 6 range. The red color is a product of iron silicate crystals forming during the cooling cycle in the kiln; it can be difficult to develop a process that gives repeatable results. Only a few people have discovered the proper combination of recipe, iron oxide percentage and firing curve (especially cooling).
Warning: Iron Red recipes will confront you will troublesome or expensive materials (e.g. bone ash, tricalcium phosphate, lithium carbonate, gerstley borate, ulexite). Getting and using these will put your dedication to the test, especially when testing produces initial failures.
Warning: High iron content causes glazes to gel, making application difficult. More water is needed to get fluidity, which causes higher drying shrinkage which leads to cracking during drying. Don't take this warning lightly, working with iron reds is working with "buckets of jelly" for glaze.
Are you sure you want to do this? If yes, then this might be a good starting point.
While it would seem logical that these glazes should have a very fluid melt and a slow cooling cycle during firing to give the red iron crystals time to grow, in actual practice, we have not been able to confirm either assumption. However, it does appear that a thick application is needed to encourage the crystallization (thus there is a danger that too much thickness will result in it running down off the ware). Experience is needed to achieve a workable thickness to be able to manage vertical surfaces. Just try to apply the glaze just thick enough that you can tolerate the amount of running.
The original recipe, from which Ravenscrag Plum Red was derived, employed Gerstley Borate to source the boron (the melter). That was a problem because it gels glaze slurries (there is already 15% iron oxide present and that gels also). Thus ulexite was instead to source the boron. That was many year ago but now, ulexite is almost impossible for potters to get. We reformulated again, this time sourcing the boron from a frit (actually two frits). Frit 3249 was used to source some of the MgO needed (this glaze has high MgO levels) since it melts so well and also sources lots of boron. There is now a little less Ravenscrag Slip to suspend the slurry, but that is not a problem, the iron will gel it.
Original development of the G2896 recipe was done to match the chemistry of Randy's Red (a popular recipe). At the time we did not do any special firing schedule to encourage the growth of the red crystals.
A GLFL test for melt flow comparing two cone 6 iron red glazes fired to and cooled quickly from cone 6. Iron reds have very fluid melts and depend on this to develop the iron red crystals that impart the color. Needless to say, they also have high LOI that generates bubbles during melting, these disrupt the flow here.
G2826X - Randy's Red Cone 5
A popular Gerstley Borate based iron-red glaze.
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|By Tony Hansen|
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