High Bridge Pottery
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Posts posted by High Bridge Pottery
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Had a look through my microscope at the glazes. The low expansion has smaller and fewer bubbles.
I did read on digital fire about holding on the way up and on the way down to reduce micro bubbles so I may try that next time and see if it actually reduces the bubbles. My guess is it wont from past experience but always worth trying.
Interesting that the purpleish stain seems to reduce the bubbles even more (or maybe the stain hides them) and the blueish stain makes the surface look a bit devitrified and must be more refractory. Going to do some experiments colouring slip instead so I shouldn't have much variation in the finished glaze surface and need a different glaze recipe for different colours.
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- Hulk and Kelly in AK
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I am very happy with the results so far
especially after being educated in cone 6-10 and thinking that's the only way to get functional/vitreous ware for a long time.
Glaze needs some more testing to make sure it's durable and resistant enough but I can't see any problems with the low bisque. Having a quick look at the cone04 limits I seem at the higher end of Silica and Alumina and hopefully a good sign.
They are all a bit thinner than I would make functional ware so the extra thickness could cause issues burning things out. It seems mostly related to Lignite so maybe there just isn't much in this Dorset ball clay to cause problems.
It would be great to develop it into a throwing body too. Adding bentonite could mess with the porosity so more testing needed for that.
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Ran a few glaze tests with the low and lowest expansion glazes from a few posts above, I also tried dropping the clay by 10% and increasing frit and petalite but they settled out within 30 min and didn't leave a nice layer on the bisque so that wasn't a good idea.
Can't see much visual difference in the surface between the low and lowest expansion glazes, need to crack out the microscope and have a closer look but I feel the low expansion is better as I don't need the talc or zinc.
I was also interested in how quickly my kiln dropped in temperature from 1100c. The first 100c dropped in 12 min, the second 100c dropped in 15 min and the third 100c dropped in 20 min. Pretty quick but it's a small kiln with a not so great lid.
Low: left has two dips of one second and right two dips of three seconds. Same for all the photos.
Lowest:
Also tried a few glaze/body stains at 3% and 7%, in the low expansion glaze. They advise sieving the glaze after adding colour but I neglected to do that to see how bad it would be. I have an orange stain too but ran out of space to attach more photos on this post and it was my least favourite.
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I didn't realise you are bridging the contacts to bypass and get a reading. I am surprised it would read room temperature as from the mV type K table 0 volts should be 0 celsius. Maybe I am missing a bit of the equation and it automatically puts 0 volts as the reference junction temperature reading.
Hopefully it is just a wiring issue and not too difficult to fix.
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Ah yes I had never really compared the two but seeing the photo now the angles are much sharper.
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This doesn't sound like a fun problem to solve
I would be interested if you still had all the old thermocouples to try swapping them around and see if any of them get the kiln working again as it sounds like a really weird problem. They don't seem to exactly be broken if you can get it to read room temperature in the bypass.
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I wasn't planning on testing a hotter bisque unless I get some issues further down the line.
There''s no exact data on the melting point I can find online other than it has a firing range 860-1060C. (1580-1940f)
Not sure if that means 860 is the melting point as I assume they are specifying when it's a usable glaze. I will add some pure frit to the next bisque to see if they are melting or not.
Are they doing a hard angle with the coils still intact or more like a front loader with what we call hairpins where the element is unwound in the middle? I had a job winding kiln elements for a while, mostly for skutt kilns and they had a similar diagonal across the brick.
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Finally got around to trying a bisque firing
thought I would try 800c (1472f) first and go hotter if needed but seems to have gone pretty well. They sound like bisque, have to wait and see how they go in glaze application. Haven't got any cones that low so not sure what cone I managed.
I always used to do 100c/h for a bisque but this time I was up around 230-250c/h with no problems, a little hold at 100c just incase. Total firing time is 4 hours. Seems like I didn't have to be so cautious previously.
Here's the graph of the firing.
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You can certainly melt glazes in there as the flux used to recover precious metals is like 75% Soda Ash and 25% silica sand, a very basic glaze to dissolve all the sulphides. Not sure the clay would survive unless it is already bisque fired slowly in another kiln.
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On 12/20/2022 at 2:11 PM, Hulk said:
Looks like 1300 °C is well over Cone 10, excepting particularly (very) "fast" heating rates...
I would say in the UK 1300c and cone 10 and are pretty much interchangeable even if most kilns wont manage 150c/h for the last 40 min of the firing up to 1300c.
On 12/19/2022 at 3:10 PM, Ceramics.np.04 said:
I would test what voltage you have in the studio over a few days and get the elements made for that voltage. The UK is supposedly 230v but I get 245V at my house so any elements designed on 230v will draw more power on 245v. Would be worse the other way around running 240v elements on 230v as then you will be down on power.
Just realised it is a front loader which tend to be more complicated than top loaders. Just a guess at the elements inside being 4 in the side walls and two in the floor. The side wall elements may all be the same spec but the floor will possibly be different.
Need to know the length of the tails (probably double what they are now so you have extra to play with/cut off, maybe 9-12") what length each bit of element is in the wall and how long each hairpin is. Also need to know what gauge the wire is, the internal diameter of the coil (maybe 1/2 or 3/4") and how they elements are wired up. Once you know how they are wired up we could possibly figure out what resistance they should be.
A bad drawing to maybe help.
- Ceramics.np.04 and Chilly
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Turns out I was trying to mix to a sg of 2.1 doing 100g water to 300g dry and needed more like 140-150g water for 300g dry ingredients. Starting off with 0.2% weight of the dry ingredients in Sodium Silicate and then adding a little more. Need to try out some Sodium Carbonate solution too and see if that helps.
Ran some tests with the Kaolin I have and they are all pretty porous using similar amounts to the ball clay. Also tried swapping Nepheline Syenite for Soda Feldspar.
Did a few tests on B0118 and it went from 0.08 - 0.44% absorption. For some reason the thin slip test tiles had higher absorption than the little slip cast glaze tower but all under 0.5%.
Mixed up 4 different glaze tests to try and poured some onto plaster. Not really glazed, more a 1g lump of glaze melted so they are all pretty thick. Opened the kiln at 200c because I figured that should be similar to a boiling/freezing test with little effort. No bisque firing so once the glaze had dried I placed onto bone dry slip.
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Medium
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The high has crazed a lot, the medium has a cracks on each side and the other two don't have any crazing. The clay already had the cracks from drying as I am just pouring slip onto plaster.
Lots of different bubbles
Total shrinkage is 10%, test bars went from 100mm wet to 94mm dry and 90mm fired. Here's the unglazed fired clay.
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Interesting stuff, sounds quite particular to get right especially at the end of a long firing.
Tried mixing up another slip with half of the Kentucky clay but it just turned into a solid mass when I tried to deflocculate it
Think I must have added too much but I thought it would settle out not turn into a solid mass.
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Tried casting a few small shapes with B0118. Got to sg 1.7 using sodium silicate, still think I need to add a little more as it had slightly gelled when I tried to pour out after 15 min. Managed to get them out the mold after 1.5 hours so seems pretty quick at drying out. White spots are a few bits of plaster as first time using and I have been learning how to get a good surface finish from the plaster. Brushing on thin before the pour seems to do the trick.
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Yea they are all 20g tests. I am pretty sure the calcium is right as the pattern seem to match across all the tests. The only scale I have accurate to 0.001g has a max weight of 20g, the next one up does 0.1g to 3000g so I feel like the smaller batches are more accurate anyway.
I will hold off buying any of the low expansion frit until I try a few glazes and see if they craze. I have what I need for the zero3 recipe as a starting point.
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7 hours ago, Kelly in AK said:
Reduction is definitely possible, has an effect, at lower temperatures. In the albums on my profile page there’s one called “ Local Anchorage Clay” that has pots fired in reduction to cone 03. They’re vitreous, 1% absorption. The reds there are from copper, no stains. The black surfaces are from iron in the clay, no stains or oxides added. I reduction cool the kiln until around 1500° F, keeps the iron black. If not, it tends toward a dark brown where unglazed. I have found that if I reduce heavily through the firing that the clay gets less predictable. I’ll have sections of the kiln with slumping, bloating, and sometimes dunting. It’s a work in progress, I have a lot to learn. I imagine a clay with less iron would be less trouble in that regard. Presently I fire a neutral to light reduction until the end, drop 100° and hold for 30 minutes in oxidation, then reduction cool till 1500.°
Never heard of reduction cooling before, I will take a look at the photos. Yea I feel like if I just put it in light reduction from say 800c (1470f) then I should be able to bring out some iron spots. Nice to hear you can get copper reds too.
I wonder if there's a cheaper way to source high alkaline frit than from pottery suppliers. When you are getting into the 20-30% frit range it does start getting pretty expensive
I don't think I can get Ferro Frit 3249 here in the UK for the zero4 glaze. There is one low expansion frit that comes with zinc and a white frit that could maybe have low expansion? Just going to need lots of testing. There's probably a few recipes I could make larger batches of and start trying out glazing.
Been looking at the results again and it does seem that adding a small amount of Calcium can help reduce porosity, or I just messed up the tests
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Sorry I didn't realise I left the third recipe in Auto Unity so it's calculating to Al2O3 unity instead of flux unity which makes it look off. The all have around 69% mole silica.
It is amazing how different clays can be even with similar chemistry. I am pondering if you can get reduction at low fire so not too particular about it being white. I don't see why not but maybe I am missing something obvious.
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Would 1 extra cone make much difference to the glazes? I was planning to use more premade underglaze/stains and then make my own glaze that goes over.
There's no other reasons than that's where the article started and 1100c is a nice number to fire to
I could lower it to cone 04 if I found glazes I wanted to use.
I am not quite sure how to put these next results in more than just a table. Do colours make it better
All I really learnt is there's not much point adding Calcium or Magnesium. Sometimes it seems to help a little but I am not sure adding 2-5% is worth it unless it imparts some better working properties.
Seems like I can use 50% Kentucky ball clay without too much of a problem with porosity, the Hyplas is not so good. Some of the tests went over the 20g limit of my accurate scales so the weights with 1 decimal place are not as accurate.
It does look like you can always add 5% Lithium Feldspar with good results and seems almost better than adding another 5% frit. I don't have a 10% frit with Nepheline Syenite to compare with but adding it mostly seems to make porosity worse.
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Ah got it yea I am aiming for cone 03. I thought you were talking about something I could add to the recipe like carbon trioxide.
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Can't see any CEC/MBI numbers on the data sheets. I can find a value for KT#1-4 on digital fire of 6.2 meq/100g
10 hours ago, Babs said:C03 is used quite a bit in Oz. Basically to vitrify some of the clay bodies so functional pots can be made from the reds and white bodies usually fired a bit lower. D' Boos had a section of glazes in her book made for that firing temp. range. The terra cotta clay went a bit browner but still ok, and vitrified. Just saying.
Babs what is C03?
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I only tried 50% kaolin in my first tests 41-44 with 20% frit. I didn't notice that much difference in drying times but I need to do some proper timed tests. The porosity did increase a bit.
I haven't quite figured out the best way to test drying times as all the clays react differently to the same amount of water. The Hymod clay needs about 25% more water than the Hyplas to get to the same consistency so do I mix to the same consistency or with the same weight of water? Seems fair to mix with the same amount of water but then one is a thick slip and one is the consistency of milk.
Yes there should be a reasonable saving in wear and tear on elements and kiln. I didn't know how to really calculate the savings per firing so left it out of my cost theory but it is an added bonus.
I only started with cone 03 after copying the idea from Tony Hansen of Digital Fire https://digitalfire.com/glossary/zero3
It could be a good idea if I want a white body to fire a bit hotter. The kiln can manage 150c/h (270f/h) to 1100c (2014f) but then it slows down a lot. After 1200c (2194f) it climbs slower than 60c/h (108f/h) and maxes out around 1260c (2300f)I hope I can find some glazes that work, there's a few suggestions in the digital fire article so I do have somewhere to start. Should get onto mixing glazes soon. Need to source some colours/stains/underglaze too, looking forward to trying out some lowfire ones.
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Thank you
a bit of method and a bit of madness I enjoy sharing what I am learning and always learn a lot from people on here. Nobody in real life wants to listen to me talk ceramics 
I think the small particles size in the Hymod is good for low porosity but bad for low drying times.
There is test 72 that has 65 Hymod, 5 Frit and 30 Nepheline Syenite that has 2.78% absorption so I would think having 10% frit instead should be around 1-1.5% absorption. I will run a test and see if the theory works.
It's interesting that 76 with 32.5 hymod 32.5 kentucky 5 frit and 30 Nepheline Syenite has 2.09% absorption. I wonder what is happening there, a little more silica and less alumina in 76 but not that different.
I was hoping the Nepheline Syenite would do a bit more melting. I have a few more tests drying adding Petalite and Dolomite with 20-30% Nepheline Syenite and 5% frit so hopefully one of those will be lower than 1.5%
I did think ball clay comes with extra silica so I might not have to add any. There's no real way of telling what CTE the clay will be. It's funny that you add silica to clay to increase expansion but add it to glazes to decrease expansion.
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Just been concentrating on lowering the porosity of the clay so haven't wanted to add any silica yet.
No idea if the glaze may shiver or craze so just leaving that until I have a clay body I am happy with. I may end up needing to add some silica but might be easier to just change the glaze if I find a body that casts well and low porosity.
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Next set of results is a mixed bag, tried adding some Nepheline Syenite and found another ball clay to try that is meant to be better for casting/drying. The Nepheline Syenite seemed to do a lot for drying time. Even at 10% the tests were pretty much dry in 40 hours instead of 80! It also makes the porosity a lot worse
but seems to get lowest around the 30% mark.
B0076 looks like it could be promising if I add some Lithium and mess with Ca/Mg a bit to get under 1.5% porosity and have better drying properties. Need to find the right balance for the good and bad of each.
The more I look at the chemistry the less I am using unity, I find it easier comparing the % analysis.
Seems there is a new button on digital fire for R2O3 unity but that still doesn't seem as useful as % analysis.
It is amazing just how much frit can change the melting characteristics. These two recipes with similarish chemistry but 3% difference in absorption.
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Depending on the frit it is around the same price to a bit cheaper to buy spodumene. Petalite is maybe half the price.
Actually you can buy 25kg of 3110 for £155 and 25kg spodumene for £286. A few places seem to be saying it is no longer available. Another site has 25kg spod for £188
The wollastonite seems to be doing strange things or I went wrong somewhere but I am pretty sure I got everything right. 10% seems to be the sweet spot. I am planning to cast but I will keep that in mind if I do try throwing.
especially after being educated in cone 6-10 and thinking that's the only way to get functional/vitreous ware for a long time.
I would be interested if you still had all the old thermocouples to try swapping them around and see if any of them get the kiln working again as it sounds like a really weird problem. They don't seem to exactly be broken if you can get it to read room temperature in the bypass.
Think I must have added too much but I thought it would settle out not turn into a solid mass.
but seems to get lowest around the 30% mark.
No problems with 04 bisque firing except flat pieces?
in Studio Operations and Making Work
Posted · Edited by High Bridge Pottery
There's a few ways you can make file size smaller without actually changing the size of an image. Different file types like .png .jpg .gif .webp all work in different ways and would give you different file sizes for the same image.
You can also change how many pixels per inch are in the image, the camera is probably around 300ppi but somewhere around 72ppi is suitable for the internet and it means there's a lot less data in the file.
When exporting images in photo editing software you can also specify how much compression you want further reducing the file size.
It all gets a bit complicated but if you go to https://cloudconvert.com
click 'select file' and choose the image you want to resize
change the 'convert to' dropdown so that you have webp selected (it doesn't have to be webp you can also stick with jpeg but the compression is a lot easier to see)
click the spanner icon and find where it says 'Quality' and input 60 into the box then click ok
Now click convert and download your image, if it is still too big reduce the number you enter into the quality box. You can go as low as 1 without being able to see much difference but your file size will be way way smaller.
Hope this helps.