High Bridge Pottery
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Posts posted by High Bridge Pottery
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I'm not convinced there's much evidence that 0.3:0.7 is the most durable ratio. I mean even in that ratio there's so many different fluxes included that there's too many variables for it to be a useful rule.
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Everybody starts off way over their head with glaze chemistry. I have been chatting about glazes on here for 10 years and I still feel things flying over my head.
I find frit and clay glazes much easier to work with and control. You do have to add flocc/deflocc and possibly a binder to dial in the working properties but it is better than having raw materials bring variable qualities to the glaze slip. The fired result may be better or worse using frit but on getting the glaze onto pottery I like a simple glaze.
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Gillespie is even worse than gerstley for gelling glaze. I say keep adding more sodium silicate for the academic exercise and find a way to use a frit instead of Gillespie Borate going forward. There's nothing that is an exact swap so you would have to do a bit of testing and rejigging of ratios.
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I'm still not convinced there's much in the flux ratio or wide agreement that 0.3:0.7 is ideal.
Stull doesn't really give a reason for choosing 0.3:0.7 in his paper and just seems to sit in the middle of Orton's limits. Even looking at borosilicate laboratory glass recipes which are pretty durable they have no RO fluxes at all in the glass. I would think if you smashed up glassware into a frit you could get a pretty durable glaze that's 1 RO:0 R2O and 10x the boron limit.
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Looks pretty old, I would try contacting kilns and furnaces https://www.kilns.co.uk
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Simple frit and clay glazes are the best.
It's an interesting question, what happens if you fire a durable cone 04 glaze to cone 6? Does it lose any durability and become high in boron?
I don't think I am looking at the same graphs, cone 10 glaze flux ratio look good from 0.6:0.4 to 0.2:0.8 R2O: RO and he even says the cone 6 no boron graph might be an error in his testing so that could be a pretty flat loss off gloss across the board. I would say his research is unconvincing about flux ratios but pretty good on boron increasing gloss durability.
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It's saying the temperature is too low to read. I would try swapping around the wires if you wired the thermocouple plug, or maybe it's a loose connection.
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So even in your smaller trials the glaze is the same thickness and settles out quickly?
Can't see much in the recipe to make it thick. Been a long time since I used zinc but I think that can thicken glaze but probably not at 3%, 15% china clay shouldn't thicken it that much either and be pretty good at suspending the rest.
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I was doing a bit of reading and stumbled across a new Stull chart. He seems to be doing the same experiment but at cone9 instead of cone11. It is interesting that the OG Stull chart is cone11 but everybody uses it at any cone.
This time he is glazing tiles and making the glaze into cones to see the deformation temperature. The solid line AB is his same “best gloss line”. The dashed line CD passes through the lowest deformation temperature in relation to Silica and the dotted line EF the same for Alumina.
He then goes on to plot it on top of the original Stull chart but misses out his new gloss line so I have done my best to add it back in blue.
It’s interesting how different the best gloss lines are. I tried adding on Silica:Alumina ratios but I haven’t had any good ideas yet. The wiggly silica line and shape of the deformation eutectic look like they tell me something but I haven’t figured anything out.
If you want to read the paper - Deformation temperatures of some porcelain glazes – R.T Stull and W. L Howat. Transactions of the American Ceramic Society Volume 16. Page 454. Copyright 1914 by Edward Orton Jr Edward Orton Jr - https://archive.org/details/transactio16amer/page/454/mode/1up
- PeterH, Hulk and Callie Beller Diesel
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I was talking to a guy at work who did some glaze leach testing with lead glazes and grapefruit juice. They were getting really high lead readings but it turned out to be the grapefruits being grown near a road and nothing to do with the lead glaze. As safe as we can make a glaze a lot of the food/drink we put in is contaminated
- Roberta12, Chilly and Kelly in AK
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There's 1000kb limit per post but most csv files should be way under that.
I would prefer the x-axis in hours and another option for easier seeing of the firing is to dash the setpoint line so it covers less of the firing curve.
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The blurb on the right says (incorrect information - see Hulk's next post)
QuoteTemperatures shown for small cones were determined using a heating rate of 300C/hr (540F/hr) in a gas fired kiln. Small cones will come close to duplicating the results of self-supporting cones if mounted upright, properly simulating the position of a self-supporting cone. Typically, small cones will deform 7-10 degrees C earlier than a self-supporting cone, so the temperature values for a self-supporting cone can be used to determine an equivalent small cone temperature by subtracting 7-10 degrees C (or 12-18 degrees F).
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13 hours ago, Kelly in AK said:
The other avenue to explore is temperature. In the range you’re firing at glaze fit can change radically within a cone, even half a cone. I say that from experience because I fire my work to cone 03, low fire. The clay is unique, and the glaze is formulated to fit, but a cone lower and it’s going to craze, a cone too high and the pots will dunt.
They had the same problem at the old place I used to work. They were selling premade bisque forms and duncans PB but if the manufacturer fired the bisque hotter than usual customers would start complaining about glazes shivering/popping off. I would try firing your bisque a cone hotter to see if that stops the crazing.
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Sodium has the highest expansion so adding soda feldspar or a high sodium frit are your easiest options to start with.
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Bigger triangle
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On 10/12/2023 at 2:48 AM, BobMagnuson said:
I was finally able to lay my hands on a copy of Hamer and Hamer "The Potters Dictionary...", and read the section on Magnesium Oxide. The amount of information they present is really quite impressive. I don't want to say they're wrong about MgO being an auxiliary flux producing "late fluidity" between 1190 and 1230 C (roughly cone 6 to 8.) I'm just looking for examples demonstrating that statement. If we can find examples where MgO is clearly an active flux, perhaps we can begin to understand how it works.
My thinking goes like this: A flux, in ceramics, is something that lowers the melting point of silica. All of the other R2O and RO oxides (Li2O, K2O, Na2O, CaO, ZnO, SrO, and BaO) do this by forming alumino-silicate eutectics melting in our typical temperature range. The MgO alumino-silicate eutectic point, on the other hand, is about 1350 C, or around cone 13 or 14.
Here's an experiment where MgO is fluxing silica and possibly better than CaO, not sure what temperature. Transactions of the American Ceramic Society Volume 17, page 236. Copyright 1915.
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I would try swapping some wollastonite for talc and frit 3195 for petalite.
Something like -
Wollastonite - 20
Talc - 10
Frit 3195 - 25
Petalite - 5
EPK - 20
Flint - 20 -
How bad is the crazing? If you post a photo and the recipe it might be easier to tweak what you have if it is close to something you like minus the crazing.
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Yup, wrong ratio from somewhere. I am amazed you get anything to cast using 10x more water than normal.
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I would try removing the additional silica from the recipe.
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Looking at the way the colour sits on the inside vs outside they have used two glazes, white all over and then dipped the top in the red.
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On 7/24/2023 at 4:45 PM, Wibbleycat said:
I have just decided to start making my own glazes and searched for a nice base recipe to start off and play with. However the recipe needs calcium borate fruit which is not available readily in the uk. I have no idea how to reformulate my recipe and what to use.
Can anyone suggest a good base glaze for me to experiment with as a start. I want a glaze with a firing range 1200-1220 that is glossy. Thanks in advance.I thought they had stopped making it but potclays seem to have it in stock. https://www.potclays.co.uk/calcium-borate-frit I feel like it has doubled in price but I haven't bought any for a long time.
If you want to look into making glazes start with limit formula https://digitalfire.com/article/limit+formulas+and+target+formulas
Gelling Glaze
in Clay and Glaze Chemistry
Posted
I would be surprised if you made it to the other side of the curve but hard to know how much some sodium silicate is.