High Bridge Pottery

It had been sat on a radiator for about 3 hours before I weighed it, although the heating wasn't on for all of that time. The clay is pretty thin (1.5-2mm) so that it probably skewing the data somewhat and making it look worse than it is. If I can get 4-6x the element life then that will certainly add up I wasn't sure how to calculate the savings with elements and kiln life so just left them out of the equations for now but reading your post it seems I will save more than I thought.

I want to figure out how much dry glaze is on the bisque so not sure you need to fire it. After posting I went and glazed one of my test glaze towers seen in a previous post. Went for the most extreme and did a 6 second dip on the outside and then poured the inside and let that sit for 6 seconds before pouring out. It's been sat on a warm radiator since then so it must be pretty dry by now but I will double check the weight tomorrow. Haven't quite narrowed down the recipe yet so no big batch to test larger items. Bisque tower - 10.165g Bisque tower with glaze - 11.795g and a difference of 1.63g Doing 1.63 / 10.165 x 100 gives me 16.04% I will try another one now with a normal 2 second dip and pour and see how that changes the %. Not sure why I didn't do that as well in the first place.

I made this comment in my first post but never really followed it up and I was certainly way off in my estimations of how much glaze impacts the cost. I did a little searching today and the only figures I could find were 10-20% of the bisque weight is the weight of glaze applied. I will have to test my own work but I thought it would be a lot less. I did a quick search for a stoneware recipe that had 25% of Kaolin, Ball clay, Feldspar and silica which came out to £1.20 a kg. I did use £1 per kg in the first calculation but changed to the £1.20 value here, I kept the total weight of ware as 4.5kg in the kiln and £3 extra for a cone 10 firing vs cone 03. I also looked back through some of my cone 10 glaze recipes that varied from £2-4 per kg so went with the lower estimate for the calculations. It seems even with 5% of the bisque weight being added as glaze you are still coming close to 10% of the cost for cone 10 and 20% of the cost for cone 03. That goes up to 25% and 50% respectively for 20% glaze. Once I get over 10% it starts becoming more expensive firing to cone 03 than to cone 10. Your prices and kiln efficiency may vary, frits seem to cost at least double in the UK compared to USA. Cone 10 clay - £1.20 per kg. Cone 10 glaze - £2 per kg. Cone 03 clay -£1.35 per kg. Cone 03 glaze - £6.35 per kg.

Not something I have seen before, I wouldn't think the clay is absorbing water and expanding as the feet seem to be affected much worse than the head and I am not sure the plaster stays wet long enough or clay really does that to much of an extent. Down under = Australia

Good to hear it worked out Not sure about the shade darker, could be due to it being higher porosity/less dense or that's what colour it looks at an 800c firing. Might be down to carbon but I am doubtful. Not sure there's a right temperature, I have read the hotter you fire the higher the COE of the clay but I guess as you are still firing the clay to 1040c then it's still the same COE. I have no data to back up the claim other than people telling me about shivering issues with premade bisque when fired hotter due to higher COE of the body. The lower glazing temperature is just chosen from what works best for the glaze I guess.

I always turn off auto-correct, causes more issues than it solves. Sorry for going off on a tangent steviepeas, let us know if you try a 700-800c biscuit and how it turns out.

Min is right, Brongniart's formula is the way to go. Grams of dry material per volume of glaze = (weight of volume of glaze - weight of volume of water) x 5/3 As long as you have a way to measure the same volume you can weigh that volume of water, weigh that volume of glaze and then minus the weight of water from the weight of glaze and then multiply by 5/3. Now you know how much dry glaze is in your sample volume to test. Don't try and fix the whole bucket at once.

He has issues with auto-correct

I feel it is similar to the way we are making metal oxides safer by forming them into a glass with silica and alumina but the stain is already doing some of that for us. I am unsure if in melting the glaze it then breaks down whatever crystal matrix they formed in the stain and releases metal oxides into the melt, I assume that is more temperature related and the hotter you go the more metal oxide is release and no different to using raw oxides except in the % of the recipe as stains seem to need less for the same colour. There is a bit on his stains page that says "These processes render them more resistant to dissolving in glaze melts, or melting themselves, compared to the metal oxides from which they are made." https://digitalfire.com/glossary/ceramic+stain

Thanks Tom, I will see if I can track it down too and try and find Brownell's paper. Looks like the Brownell is still in copyright until 2063 so it may be difficult to source. I think I may have found the experiment that is referenced in the 950c-1150c temperature or at least one of them. Seems which sulphur compound you have changes the temperature as a brick maker talks about iron sulphide releasing gas up to 900c. https://archive.org/details/transactionsofam13colu/page/689/mode/1up

I would say in general colouring the clay and glazing with a transparent gloss is going to be the safer option for the end user but there's so many variables in pottery that there's no one size fits all answer. https://digitalfire.com/picture/L9hivqD172 On this page Tony says "Stains are inherently much safer to use than raw metal oxide colorants because they are sintered as colorant/stabilizer blends. And much less is needed." So it is possible to make a black/coloured glaze that's food safe but it requires testing.

You will be much better off painting on the coloured slip when the clay is leather hard. If you paint it onto bisque it will probably crack and fall off, I tried this recently and you can see from the results that painting onto leather hard clay was much more successful that onto bisque. There is probably some way to get it to work but it is much easier to paint onto leather hard as they are both shrinking at the same rate. You might get better results so probably worth testing as parian porcelain is much more glaze like than my clay. Looking at a few recipes there's only 20% kaolin clay in the parian instead of 60% ball clay in my slip.

Sorry I probably didn't explain it the best. I don't mean paint a layer and let it set and then do the mold. Once I have the mold box set up and ready to cast in plaster I will weigh out my plaster and cold water in the right ratio. Add the plaster to the water and let it sit for 1-2 min then gently mix with my hand for 2-3 minutes until I feel it slightly thicken. Now that I am ready to pour the plaster into the box I will use a brush dipped into the plaster and paint onto the form to get into all the detail and then pour the plaster into the box to cast the form. I found if I don't brush the plaster onto the form and just pour the plaster into the box I end up with bubbles on the casting surface. I got the idea from this video, he is not using a box to cast the form and lets the plaster thicken up after brushing a lot more than I would to pour the mold but the brushing idea is the same. Later on in the video when he is pouring the plaster head you can see the kind of consistency I would pour the mold at after brushing it onto the form to capture all the detail.

There's no problem using a plasticine form to cast plaster molds from, it could be a bit tricky to block off areas for multi-part molds but some normal clay will probably remove pretty well when you come to pour the next section. I did read some plasticine can mess with silicone curing but nothing about plaster. My only advice is to get a soft natural hair brush and paint on a thin layer before you do the main pour, that's the only way I found to get rid of most/all air bubbles on the surface. Tried different surfactants and different soaking times and mixing times and they all seemed to trap bubbles on the surface no matter what.

I wasn't trying to claim it is safe to fire the kiln or good of the electrician to miss the inconsistencies just that a 30 amp socket/plug on a 40 amp breaker fits the code.

Isn't 25% greater than 30 amps 37.5 amps? I know it doesn't match the kilns amps but it seems within the code and the electrician is oblivious to how much power the kiln is using?

Had a read of the firing section but he seems to be agreeing that 800c is fine for carbon burnout. "This process requires at least the dull red heat of 700c (1292f) and reaches its climax around 800c (1472f). Most of the carbon has been burnt out by 900c (1652f) but some sulphur lingers until 1100c or 1150c (2012f or 2102f). "Some potters slow down the rate of firing or try to hold the temperature steady at 800c (1472f) for a couple of hours." I am not sure I agree about the sulphur being gone from 1150c onwards after seeing the glass batch with 1% sodium sulphate releasing SO2 gas from 1000c to 1550c but I would be interested to find the experiments that have been used for the ceramics data. These numbers must have come from somewhere.

Ah seems like I misread your previous post and you are talking about noise from the elements/kiln when it gets hotter and not the relays. My mistake. Hopefully the new controller will stop this issue.

I can't see many problems with doing a low bisque although the general instructions are to do a higher bisque and lower glaze firing. I assume that is to get absorption lower and a higher COE in the body to match with the glaze. Doing a low bisque could give you crazing issues with the glaze. It is certainly possible to make functional ware but you rely on the glaze completely covering all the clay to stop it absorbing water. Sometimes earthenware clay is better suited to being outside as it has good thermal shock characteristics with the body being more open. @Babs I wish we didn't use miles in the UK but that one has stuck around. It's funny, fuel is sold by the litre but everybody talks in miles per gallon

I found it interesting after reading a few of the American Ceramics Society's work that they all seem to use celcius. What happened What's the title of that work? I would be interested to read it.

Yea probably not the best idea Thanks Neil, didn't know it could get worse at higher temperatures, I thought the relays would always make the same amount of noise.

You can twist the wires, that means the noise on each line cancel each other out. Used to do it with phone lines. If it was noise wouldn't it be happening throughout the firing?

What kind of clay are you working with? You said "I would fire high, say at 1040 for glazing," which is cone 05/04 and pretty low for a glaze firing unless it's earthenware clay.

I find elements only hum or buzz at lower temperatures anyway so I wouldn't worry about that. I know in my D.I.Y controller when it gets up to higher temperatures it starts reporting a lot of 'shorts to ground' because of the plasma generated in the kiln at higher temperatures. At least that is what I read.

I'm not so convinced that 800c is really really low for carbon burnout. There doesn't seem to be a real consensus around temperature. Thickness, time, air flow and the amount of carbon present all play their part. You could bisque to 1000c with rubbish air flow and thick pieces quickly and get less burnout that a slow bisque with good air flow to 800c. Digital fire seems to say anywhere from 370c-950c others say that naturally occurring carbon in clay burns off at higher temperatures: up to 1200 F to 1400 F (650 to 760c) so I think temperature is just one variable.