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High Bridge Pottery

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Posts posted by High Bridge Pottery

  1. 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.

  2. 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 :unsure: 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.

     

  3. 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.

  4. 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.

  5. 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.

    804389415_ClayGlazeCalculations.jpg.0c535bf2a636e4f3cdeb5c64d3f15b7c.jpg

  6. Good to hear it worked out :D 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.

  7. 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.

     

  8. 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

  9. 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

    741767717_CalciumSulphate.jpg.0860191aa6c18c51b06ce58c332c91c6.jpg

    800544074_CalciumSulphate2.jpg.24bdeb629eaec8107d3025aa50ceb7a8.jpg

    1655765289_CalciumSulphate3.jpg.c18edb1a4fcc1369c0049cb8db30b2b6.jpg

     

     

  10. 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.

  11. 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.  

     

    IMG_20230223_092702_9.jpg.110b94fadcec3143f94ccd03f60f9796.jpg

     

     

  12. 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.

     

  13. 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.

  14. 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. 

  15. 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 :blink:

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