Potters & Sculptors - Making Rock from Mud
I use titanium at 93% and RIO 7%, which for my glazes work fine, if it's too dark but almost right try spanish red iron which is a bit weaker than rio,
I've calcined dark rutile and it came out light in color but did not test for glaze color changes
George, it would be nearly impossible to replicate the differences between light and dark rutile. I think the light is something like 2-3% iron, and the dark runs 10-15% Fe2O3. I think it would be easier to reformulate the recipe to adjust the iron levels proportionately. Although rutile iron levels are inconsistent, pending the vein they are milling on any given day.
We have three different bags of rutile and each one looks different. One produces a milky white like titanium dioxide, but none of them produce the white streamers in clear glazes which our very old orignial bag created. After the third bag I gave up trying to find the exact same material. We have light and dark and granular.
All I can say is the material is highly variable. I've often wondered if our original bag was somewhat more coarsely ground. All I can think to do is ask Ron Hesselberth whose non-potter career was overseeing the production of titanium dioxide from rutile ore. If any one has any ideas it would be he.
You know rutile is used as a flux compound in welding. If I remember you can buy rutile dry flux from welding supply store or check to see if it's the same as they sell. A fellow potter may have some old stock like you have, will ask him today.
Is it you want the effect the old version produces?
My main use is for the glaze Nutmeg. It formerly was a light tan breaking red brown. When used as a base it would soften and variegate the over glaze. Here it is under variegated slate blue on the lower exterior of this cup. I like it for softening green as well. Current batches are almost like saturated iron glazes, going dark tan only in the thickest areas on the verge of running.
There were rutile blue glazes that had no other colorant. About 6% would make a hare's fur blue where thicker.
Tony Hanson at digitalfire has a discussion on rutile blues and slip clays, a good read.
I think you might try a line test of rutile and titanium 50/50 might be a good start and maybe a bit of flux like 3134 to aid in the movement if needed. You may also try adding some zircopax to the nutmeg and/or switching the rio to spanish red iron ox.just a few thoughts Wyndham
I always thought the difference between light and dark rutile was that light rutile had been calcined. The reason for calcining was to lessen issues of pinholes/bubbles in the glaze when higher percentages of rutile were used. I have taken dark rutile, run it through bisque and it did appear much lighter.
I'm sure changes in mine locations and processing have also changed the material over time, as has most all other natural materials we use.
Yes that's the difference between light and dark rutile. But which ever you purchase it's a natural ore so it varies greatly from one purchase to the next.
If you find rutile you like, you'd best buy a lifetime supply because you'll never have access to that type of rutile ever again. After 7 years we were almost out of our existing bag so bought rutile, and again from 3 different suppliers and each one fires into a different look - none of them like the original.
By adjusting the amount added I could get closer to the look of the original, but it's not exactly the same.
Tony Hansen at Digital Fire has an extensive article on rutile (and almost every other ceramic material) to fill in the gaps in our understanding.
Quoting from the article,
"Rutile powder, although its color makes it appear to be a very crude ground mineral, normally contains 90%+ titanium dioxide. However this does not mean that you can use a 90% titanium:10% iron mix and get the same result in a ceramic glaze (obviously line blending would be needed to match the amount of iron). The mineralogy and significant other impurities in rutile are a major factor in the way it acts in glazes and are not easily duplicated using a blend of other things. Sometimes the special effects that rutile produces in glazes are also partly a product of a coarser grade (larger particle size). These likewise cannot be easily duplicated by more refined materials. Unfortunately the trend at some mining operations (at least in Australia) is to fine grind the rutile on-site, making it more difficult for ceramic operations to obtain the coarser grades."
Axner shows three grades of Rutile. At one time they showed the iron content of each in the description: they have since changed it.
Hi, I was trying to get light rutile for a long time. One day somebody said: It is the impurities that make Rutile so dark, fire it for at least 800 degrees C to get the impurities out. And bingo: after this firing I have light Rutile!!!