High Bridge Pottery
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Posts posted by High Bridge Pottery
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FeO is certainly a much more powerful flux but Fe2O3 can help with melting. I don't think you are missing anything entirely, there's quite a fine line/small area of chemistry that will make a good matte glaze as you have found out by removing the iron oxide.
All the B2O3 Fe2O3 Al2O3 oxides are a bit weird and can act as fluxes and stabilisers. Can be called intermediate oxides or amphoteric oxides.
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Dunno if it would work but you could put them in a box with an ozone generator, just be careful not to breathe the O3.
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Made up a 1kg batch with 1% bentonite added to see what it's like to throw with. Very rusty on my throwing so left it quite soft to make it easier. I tried a bowl first but that ended up flopping down so I put that on a plaster batt to dry out some more. First cylinder on the right was with the same really soft clay and the left one is the rewedged bowl. Seemed to throw about as well as the cone10 clay I used to work with. Might try another 1kg batch without the added bentonite and see if I can notice much difference. Can't find my good wood tool for cutting in the base
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That's true you could make them all 1.44 ohms instead on 240v and get 16.6amps and a 4kw kiln instead of 15 amps and 3.6kw. Looks like the controller has a 30amp relay so that shouldn't be a problem on the relay either and give you additional room for wear.
What size is the breaker on the kiln? 16.6amps +25% does just take you over 20 amps but it might be bigger anyway.
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They are definitely worn out, you need to get new ones made up with a resistance of 1.6 ohms per element. They will have been that resistance to start with but over time they wear out and increase their resistance.
It's not that you can't supply enough power to the kiln it is just that as they wear the resistance increases and the amps will decrease meaning you are getting less power to the kiln. Amps are just a function of voltage divided by resistance, we can't change how much voltage comes in the wires so by changing the resistance you can increase how many amps the kiln will draw. It's one of the reasons we have breakers and fuses because things can go wrong and many more amps can be supplied through wires not rated to handle that power if the resistance drops for some reason like a short circuit.
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I always found it pretty tricky to get an accurate reading from long lengths of wire as it liked to coil back onto itself and give weird readings by shorting out sections. Measuring the length with a stick cut to 1 meter and adding 5-10cm extra for the tails seemed to work pretty well. Wearing safety glasses is a good choice when measuring and winding as it likes to spring back and swing for your eyes.
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Thank you for sharing. Doing the maths and either pretending the water doesn't exist in the weight after dipping or assuming the bisque weight it the same as fired clay weight and LOI is 6.5% you are somewhere in between 5-10% dry glaze for the weight of bisque. I always felt it would be 1% or less but never ran any tests to prove that to myself before now. I think for a glaze that could be thinner maybe 3% is the lower limit.
- Callie Beller Diesel and Min
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Ok think I got it right this time and didn't confuse my metric and imperial. 12 gauge comes out a bit under the 5-7 length but I wouldn't worry as it is in the 2-3 stretch length.
12 awg - 0.1340 ohm per foot, 2.052mm diameter.
d = 0.002052m
D = 0.007052m8 / 0.1340 = 18.197m (59.7f)
18.197 / (0.007052 x pi) = 821 turns
821 x 0.002052 = 1.685m (5.53f) wound lengthLooking at 13 awg you can probably just get away with stretching it a bit further than recommended.
13 awg - 0.1690 ohm per foot, 1.8288mm diameter
d = 0.0018288m
D = 0.0068288m8 / 0.1690 = 14.429m (47.34f)
14.429 / (0.0068288 x pi) = 673
673 x 0.0018288 = 1.231m (4.04f) -
You are right, sorry about that. I also noticed in the video he is adding the wire thickness to get the D value so I will rerun the calculations and not mix up feet and meters!
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16 awg might work if D is 5mm, couldn't find any exact numbers on what the internal diameter is but they look about 5mm. Can you post what the internal diameter is?
8 / 0.3388 = 23.613m
23.613 / (0.005 x Pi) = 1503 windings
1503 x 0.001291 = 1.94 meter (6.36 feet) length unstretched which you can stretch to 15 feet and is within the 2-3 stretch ratio
4.57 meter (15 feet) x 5 = 22.85 meter so under the 23.613 length of straight wire.
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38 minutes ago, elenab said:
If my math is not wrong, can the new coil go around the kiln only once instead of 3 times? But I believe something is wrong with my math, it can't be that ( for the same amount of ohms) the longer the wire the thicker it is.
Thicker wire has less resistance per ft so for the same resistance you will need a longer piece of thicker wire to get the same resistance as thinner wire. Not sure that you need 5-7x longer than the coil length as it depends what sized rod you are winding it around and what gauge wire you are using. I would just measure what gauge you have in the kiln now and go with something similar. If you need to stretch it to 15 feet then then it's probably going to be around 5 feet before you stretch it and need a mandrel (rod) at least that size and matches what internal diameter you need for the coil.
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Once it's neutralised with bicarb isn't it fine to pour down the sink as it's just salty water?
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1 hour ago, Callie Beller Diesel said:
When I used the Brongniart calculator to figure out the dry weight of 10 ml of a glaze with sg 1.4, I got 4.6g.
This one? https://pietermostert.github.io/SG_calc/html/brongniart.html it gives me 6.5g as the answer. If it's a sg of 1.4 then 10ml should weigh 14g? and 10ml of water is 10g? I use the post on clay art so (14-10) x 5/3 = 6.667 Unless I am wrong about the glaze weighing 14g.
I tested the glazed bisque again this afternoon and got 7.25% of bisque weight for a 2 second dip and 14.75% for the 6 second dip. Some off the loss from my previous value in % of the 6 second may be from touching it and a bit of water as my glaze is a little dusty on the surface and comes off easily.
I would be interested if anybody wants to try out weighing bisque then glazing and drying to see what values they get.
Finally got some HCl to try dissolving these crystals, just got to work out how to dry and weigh again after sitting in HCl as I am assuming it wont all dissolve. Need to read about safety and disposal, guess I should neutralise it before disposal and figure out a way to catch the crystals again. I don't want to ruin the mesh in my sieve so maybe coffee filter paper.
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45 minutes ago, Min said:
Are the figures of 10-20% for thinly cast wares, thrown pots, sculpture or just a general average?
I found this statement here - https://www.oldforgecreations.co.uk/blog/pricing-your-pottery
" To calculate the weight of glaze - weigh a dry piece before you glaze it, glaze it as normal, leave it to dry fully, weigh it again. The difference is the glaze weight. If you can't be bothered, just assume it's a fraction of the clay weight. 10-20% should do it."
So it's not really that specific or accurate
but I couldn't find anything else giving any better ideas.
Maybe it is better to think about surface area than weight of the bisque, I did just find this talking about spraying glaze - https://digitalfire.com/4sight/datasheets/GlazeSprayingForCraftPotter.pdf
"Now the second thing you need to measure, is how much glaze is appropriate for a given area of pot. This is easiest to explain with an example. One much-used glaze is a dark rich temmoku which we mix to SG=1.4 , at which it is just right for dipping mugs and bowls and such. After spraying this glaze umpteen times onto various pots (chiefly big platters and casseroles, not convenient to dip) we’ve decided that 10 mL of this glaze is just right for an area of 100 square centimetres"
I guess that means for 100 square centimeters they are using 6.5-7g of dry glaze. If we have a cylinder 8cm diameter and 10cm height and pretend the inside has the same surface area as the outside that make ~700cm2 and 45.5g of dry glaze. Could probably make a mug that size from 500g/1lb of clay maybe a bit more so it still seems around 5-10% of the weight of bisque could be used to estimate dry glaze.
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8 minutes ago, Bill Kielb said:
How much of that is water? Maybe not a bunch…..
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.
3 minutes ago, neilestrick said:There are other cost savings with firing to lower temps besides just the cost of electricity, big time if you're firing to cone 10 in the electric kiln.
- Extended element life. Firing just 04 will give you 2-3x the element life versus glazing at 5/6, double that vs cone 10.
- Longer kiln life
- Longer kiln furniture life
- Less energy removing excess heat from the studio (if you're running AC or fans)
Plus you can increase output with shorter firing schedules, and it's better for the planet.
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.
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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.
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On 11/24/2022 at 2:21 PM, High Bridge Pottery said:
Never did any working out for how much more the glaze costs so maybe that will make things look worse but hopefully it is such a small % total it doesn't make much difference.
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.
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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
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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.
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21 hours ago, glazenerd said:
Orton — auto-correct once again.
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.
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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.
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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
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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
but I couldn't find anything else giving any better ideas.
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.
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.
Confused Newbie. Cone 6 base matte not firing as expected
in Clay and Glaze Chemistry
Posted
It's the same way Silica doesn't melt until 1700c by itself but by having a mixture of oxides you end up with a lower melting point.
A good example is a mix of Silica and Alumina. "The melting point of silica is 1710c and that of alumina is 2050c. At the eutectic point, when 10% alumina and 90% silica are mixed, the melting point is only 1545c. All other ratios of silica and alumina have higher melting points."
https://lindabloomfield.co.uk/wp-content/uploads/2017/04/Bloomfield_May17CM.pdf