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Adding 20 SiO2 only brings the SiO2:Al2O3 ratio up to 6.1:1 (using Insight data) so I'm not surprised it isn't very glossy but it does look like it's moved it a bit towards a gloss. There is going to be a limit to the amount of SiO2 the fluxes can incorporate before running into the issues of undissolved silica itself causing matting. Re the quote from Hansen above, is he talking low fire when he says "At lower temperatures the matting mechanism of MgO is that it simply stiffens and opacifies the glaze due to its refractory nature."?

MgO is in the role of a flux, glaze in question is a high alumina matte, adding silica doesn't negate the fluxing of the MgO it moves the SiO:Al2O3 ratio enough to move from a matte to a gloss due to the increase of SiO. edit: was the kiln slow cooled?

@Pyewackette, +1 for just using a very light spraying of flashing slip. Also, I would pour the liner glaze not spray it. When you spray inside a form such as a deep bowl or vessel there is a huge amount of glaze blowback coming back out at you. I'ld spritz the inside of the (bisqued) pot with water at the bottom and lower part of the wall where the glaze will be thicker and do a quick pour of glaze of about 1/3 the volume of the pot then very quickly pick it up and swirl and dump the glaze out to coat the inside. If you are using the same glaze for part of the top / outside then quickly dip that area after doing the liner. I'ld also hold this pot back from your first soda firing. I would suggest taking some less precious pots, take some really good notes on what slips are on each pot, application details, where they are in the kiln etc. Learn as much as you can from the first firing, how other peoples pots turned out and any details you can get about those pots and the firing then apply that knowledge to the slips/glazes/placement/firing of this large one.

Did you do this with and without the MgO or was it just to demo the SiO:Al2O3 ratio effect on gloss levels?

I have no idea, if anyone knows could they post?

I'm not going to comment on the durability of this glaze but I did rework it to contain nearly double the epk and got rid of the nepsy and supplied what it did with a frit plus Minspar. Should suspend a lot better with less bentonite needed (if any at all). If you try it Morgan please just test a small amount first, what looks good on paper and all that. edit: if you test this and find it's too fluid (because of the frit plus tiny bit of boron) then it might need an extra 1 or 2% of EPK

A well fitting liner glaze is usually always used inside soda, salt and wood firings for all functional pots. Wouldn't want to drink out of mug without a glaze inside it. If the liner glaze has a significant fit difference from what lands up on the outside of the pot then the pot can dunt / crack. If you glaze the entire pot and then it's hit with the soda it can very well cause excessive running of the glaze due to the extra fluxing of the sodium. If you just glaze the top of the pot and leave a lot of room for it to run it should be fine. It depends on how much soda the pot is hit with. Nope, flashing slips don't all look the same. I used a few on the vase below. The lighter orange one has titanium in it, the red doesn't.

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If you do want to give standing while throwing a go a less expensive way to go than the Brent extension legs is to use legs from an old table. My mum's old sewing table had solid wood legs, I repurposed them to be my wheel extension legs. (I use a Bailey which has the pipe legs like the Brent) If you can find an old table with solid legs at a thrift store or wherever that fit inside the pipe legs then remove them from the table, figure out the height you want then drill a hole through both leg and wheel leg and bolt them together. Since my Bailey is like Brent wheels and there isn't room to sit the foot pedal next to the wheel and use a hand to control it we added a piece of redi-rod to the side of the foot pedal then screwed a knob on top.

Throwing out a thought that might simplify this debate. Q: what colour response does cobalt give in a high magnesium glaze? (including midrange) A: shades of purple If the magnesium wasn't entering the melt as a flux then no purple. Dave Finkelnburg article on cobalt purples and magnesium.

@Kelly in AK, John Glick's article is a good one, it's what got me started on standing up while throwing. When I made the switch to standing I had lower back pain but my main reason was pififormus syndrome from leaning against my throwing stool and the edge of it pressing into my rear end. Took a few years for the sciatic leg pain from the piriformus injury to go away but the lower back pain was gone long before that. I don't use a back support/post like in Glick's article, just stand up and brace my arms against myself while centering.

Throwing out another alternative, the Hyde Stir Whip. I had problems with my Jiffy mixer scraping plastic from the sides of the buckets over time so I started using a Stir Whip. I've it for about 9 months, so far so good. I use an electric drill, haven't had splashing problems. It takes a few seconds longer to get the mass of glaze moving but once it is moving it mixes really well.

Hi Gail and welcome to the Forum! In an ideal world we would use a low expansion claybody paired with a well fitting low expansion glaze for ovenware. In reality a lot of people don't do this yet still make ovenware, usually when selling the pots they add a cavet to place the filled pot in a cold oven then turn the oven on along with other advice like using it in the middle of the oven and taking the design into consideration. (even wall thickness, no pooled glazes, no sharp corners etc) To make a midrange low expansion clay manufacturers use materials like spodumene, pyrophillite and carefully control the silica. Adding these to an existing body is going to alter the maturity temp/cone of the clay. I would suggest asking your supplier which claybody they recommend for ovenware, ask the manufacturer if the supplier isn't sure.

I think the flax “paper clay” is from your side of the pond Peter. I have a friend who went to a workshop in England a number of years ago who made a flax paper clay pot, couldn’t find any suppliers of it over on this side of the pond when we looked for it. I wonder why it never has become a thing over here? Cost maybe? I know some handmade paper supply places carry flax fibres, it would be a good experiment to see how flax vs paper clay compared in regards to rotting.

I would ask the person who fires the kilns to put it through the bisque firing as it is (with the glaze on it) then put it through the glaze firing. Saves having to do slow ramps for an entire glaze load. Welcome to the Forum!

Hi and welcome to the Forum! I would toss any scrap or waste clay that has sodium silicate on it. It’s such a strong deflocculant you don’t want to mix it with your reclaim.

Greenbarn in Surrey BC carries it also. $79.73 / 50lb box. It might not be either ilmenite or granular manganese in IMCO's Dragon Fruit, could be a black fritted material like Georgie's uses in their Pioneer Dark with speckles.

Oil based wax resist can still be thinned with water but they work much better than the water based wax resists when using overtop of a glaze. Won't get the lifting issue that can happen with water based resists. Aftosa is one. https://www.baileypottery.com/c-042-104-c.html They won't ship if the weather is freezing.

@Kathryn Howlett, whatever testing you do I would strongly suggest putting a waster cookie or shallow dish under your glaze tests in case you get glaze running onto the shelves. The waster cookie or dish can be reused.

You can run into problems of dunting (a type of cracking of the pot) when using glazes that have fairly different coefficients of expansion (COE). The crazed glaze is going to have a higher COE than a non crazed liner glaze. You could use the same crazed glaze both inside and outside the vase but make sure the clay you use has a low enough absorption to not leak. Absorption should be below 1 - 1.5% for functional work, including vases. Link here for how to test for absorption if you need it. https://ceramicartsnetwork.org/daily/article/Testing-1-2-3-How-to-Test-Clay-Bodies-to-Find-the-Right-Sculpture-or-Pottery-Clay-for-Your-Work#:~:text=The weight calculation test is,weight of the absorbed water.

Do you have the recipe(s) for the dipping glazes? Are you permitted to underfire the kiln or are there other peoples work in the same kiln firing? Are you making functional work?

Not too confident it would be sufficient to take a heavily crazed glaze into one that is a lot less crazed but what you could try is adding some silica to it. There isn't always "room" in a formula to dissolve more silica but if the glaze is able to then this will reduce crazing. Given we don't have the recipe it's going to be all trial and error as to how much the crazing will be reduced and if the glaze is able to dissolve it. Is the glaze a brushing glaze or dry glaze that is made into a dipping glaze? Either way I would suggest drying out some of the glaze and weigh out 100 grams of it. To this add 5 grams of silica, then reconstitute and sieve the glaze. Apply it to a test tile then add another 2.5 grams of silica, sieve and apply to another test tile. Repeat one more time with another 2.5 grams of silica. This test isn't very accurate since each time you are applying the glaze you are removing some of the inital 100 grams but it should give you somewhere to start. edit: have you tried the glaze on a different claybody? Crazing or non-crazing is a glaze fit issue with the claybody.

Cement floor in all areas I work in. I use a really good quality anti fatigue mat when standing on for throwing. I wear supportive shoes with good insoles, think that helps. It's T-7 - T-9 that I'm getting a lot of pain with for the past few weeks.

Given the recent resurrection of this thread I made up a chart so it's easier to make a comparison of the various spars and their compositions. "Kaolin Washer"refers to the analysis he had done on Pacer’s Custer done 2017, link to it here. Pacer refers to Custer spar from Pacer and Roy refers to Ron Roy’s data from his chart posted above. In addition to the significant K2O and Na2O differences between the analyses what jumps out at me is the much lower Fe2O3 (and CaO but not significant) in the Pacer Custer in the 2020 and 2021 analyses.

Hi Ron, I also have another analysis that was done in June 2021 for a clay manufacturer, I'll email that to you. My supply of custer is old stock so no point in sending you a sample of that. @Dick White, could you send a sample of yours?