cwells

Is that assuming no boron in the glaze? We looked earlier at glazes with 0.62 alumina that fired to cones 4-7.

Thanks I appreciate the long post. That definitely reinforced that I'm getting on the right track to start making proper glazes. There's been so much information i've tried to absorb from the internet over the last couple of weeks. How I got to it recipe wise, or how I decided on that level? Recipe wise, it has a fair bit of EPK/calcined EPK (it got real gummy without calcined kaolin especially with all the iron). For reasoning, to be honest, I wanted the glaze to be firmly into the matte region. I was also trying to not get close to the craze region as I've learned the craze region can move up the y axis depending on the glaze, so I was hoping to have it firmly there. I use a shared kiln that fires twice a week at cone 6, so not much room for playing around, and won't be able to test the glaze at a higher cone. I still find it odd that the same base seemed to have matured with the fe2o3 but not without. I'll try to remake the base and fire it once more incase I made a mistake somewhere. If it's still clearly underfired I'll lower the al2o3 in one test and increase the boron in another and try again. The glaze is kinda pretty with the colorants and seems to be fairly resistant. Planning on making some pots that look cast iron. I'll at least keep using it as is, just quite confused about iron. Red iron oxide seems to have so much conflicting information about how it behaves in a glaze. If you can see the images below, the left one has been dug at by keys and pin tools and left in detergent and acids overnight and seems fine. The one on the right is the base glaze, that chips off easily

My reasonings on the ratios are that the stull chart shows true mattes seem to sit below a 1:5 alumina to silica ratio. ~0.15 boron and a 0.30:0.70 alkali:alkaline ratio look to make the most durable glass when fired at cone 6 as per an NCECA talk about glaze durability I watched, which included a chart of measured durability of a glaze with varying boron levels and alkali:alkaline ratios at cone 6. Every other tile melted just fine at cone 6, just this one without iron. Maybe it just didn't get mixed well enough. Perhaps it's a mistake I made somewhere when making the batch since all of the other tiles with colorants matured. I was looking at glazy for recipes with similar ratios and found https://glazy.org/recipes/147183 which is very similar to what I came up with and it melts at cones 4-7 in the semi matte region of the stull chart.

I'm so confused by that with its melting point being so high. Guess that comes with being new, can't understand everything at once. Thanks! Thanks No, neither of these two tiles did not have cobalt. I did a test earlier with the same base of 8% iron and 1% cobalt to get a dark black.

Hi all, I started potting as a hobby about a year ago. A couple weeks ago, I got into glaze making and decided to try to make my own matte base as an exercise, after consuming all of the free information I could. It seemed to have worked well, as I got the desired effect of a cone 6 matte black. I followed all of the information I could find, and made sure it was 0.3:0.7 alkali:alkaline metals, 0.16 boron, and it charted on the stull map as a matte at 0.74 alumina and 2.54 silica. At that time, I did not fire a test tile without the colorants. I replaced some of the EPK with calcined EPK as I was having some crawling issues with application, and made two more test tiles. Where I'm confused, i tried to fire the base without the added colorants, and fired the two test tiles together. One with 8% yellow iron oxide, one with no iron. The one with the iron fired nicely and seems durable to the eye (added cobalt didnt seem to dissolve when soaked in acids, glass didn't seem to erode in bases, and doesn't scratch or chip). The one without the iron is a dry brittle glaze that I can flake off with a pin tool, so I assume it's underfired. From what I understand, in oxidation fe2o3 should have a higher melting point than cone 6, and doesn't give off it's oxygen until cone 7-8 to become feo. I'm not sure where my misunderstanding lays, does iron oxide act as a flux in cone 6 oxidation, or am I missing something else entirely? Thanks!