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#1 Celia UK

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Posted 21 December 2012 - 05:04 PM

I am fairly new to ceramics, very new to firing my own work. Have small 40 litre electric kiln - make small thrown pieces. I mostly use smooth white earthenware and clear glaze with oxides added or over underglaze colours. I have fired various clays at the same time e.g. St Thomas white, and different White Earthenwares, I mix the glaze from powder and sometimes add oxides. I DO record details of all firings but the outcomes have been so inconsistent I haven't been able to identify a) ideal bisque firing temp and B) glaze firing temp. Some pieces made of same clay body and with same glaze, are not equally successful. I have had 'pinging' problem with glaze cracking up to a week after firing on some pieces, (I understand this is a glaze fit issue), but I don't understand why this happens on some pots and not others? Everything is always bone dry before bisque firing and I leave the kiln until it is cool before opening. Have bisqued at 06 and glazed at 05: bisqued at 04 and glazed at 05 (read somewhere to bisque higher was better to ensure nothing nasty came through glaze....but generally others say glaze higher!)

Form is more important to me than the decorative effects. I LOVE my unglazed, bisqued, smooth white earthenware, and am exploring subtle oxide additions for some pieces. My best pieces recently, were clear glazed on the inside and unglazed on the outside - no 'pinking' on these pieces, but 2 that were glazed inside and out now have long cracks in the glaze. It's so frustrating when so much work has gone into the pieces - throwing, carving, incising, building on etc. then a successful bisque, all to be lost in the glaze firing! ANY ADVICE/SUGGESTIONS would be most welcome.

#2 Marcia Selsor

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Posted 21 December 2012 - 05:38 PM

Actually, you might try bisque firing higher and glaze at the same temperature. For Majolica some people actually bisque to ^03 or 02 and glaze at ^04 to avoid crazing in the majolica. You looked like you are doing something similar by bisque firing at 04 and glaze at 05.
Maybe try bisque firing higher still at 03 or 02. And slow cool. Don't open the kiln until it is at room temperature. Where are you located...I don't think you are in the US. since you describe your kiln size in liters. Possibly others on this forum know of more compatible clay and glaze closer to where you are located.


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#3 Celia UK

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Posted 21 December 2012 - 06:05 PM

Actually, you might try bisque firing higher and glaze at the same temperature. For Majolica some people actually bisque to ^03 or 02 and glaze at ^04 to avoid crazing in the majolica. You looked like you are doing something similar by bisque firing at 04 and glaze at 05.
Maybe try bisque firing higher still at 03 or 02. And slow cool. Don't open the kiln until it is at room temperature. Where are you located...I don't think you are in the US. since you describe your kiln size in liters. Possibly others on this forum know of more compatible clay and glaze closer to where you are located.


Marcia



#4 Celia UK

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Posted 21 December 2012 - 06:15 PM

Thanks Marcia. Think I need to narrow down my testing to reduce the variables. Once I have a new batch I think I'll do some systematic test fires and just change 1 variable at a time - as a former teacher, I should know that that's good scientific investigative practice! It's only the cost factor of firing the kiln that stops me doing this - want to get on with making final pieces! I guess this is a case of more haste, less speed and time taken at this stage will pay dividends in the long run.

As generally i only want a thin glaze then bisquing higher would help with that I believe, by making the clay less porous?

I am in the UK - well spotted! It always surprises me that the US hasn't ever gone metric - I thought we were the dinosaurs! I have just joined a local Potter's Group, so I will be able to pick other members' brains there too, in the new year.

In the meantime I'll keep my fingers crossed until I open the latest glaze firing, in the morning.

#5 SEWSart

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Posted 21 December 2012 - 08:44 PM

I am fairly new to ceramics, very new to firing my own work. Have small 40 litre electric kiln - make small thrown pieces. I mostly use smooth white earthenware and clear glaze with oxides added or over underglaze colours. I have fired various clays at the same time e.g. St Thomas white, and different White Earthenwares, I mix the glaze from powder and sometimes add oxides. I DO record details of all firings but the outcomes have been so inconsistent I haven't been able to identify a) ideal bisque firing temp and Posted Image glaze firing temp. Some pieces made of same clay body and with same glaze, are not equally successful. I have had 'pinging' problem with glaze cracking up to a week after firing on some pieces, (I understand this is a glaze fit issue), but I don't understand why this happens on some pots and not others? Everything is always bone dry before bisque firing and I leave the kiln until it is cool before opening. Have bisqued at 06 and glazed at 05: bisqued at 04 and glazed at 05 (read somewhere to bisque higher was better to ensure nothing nasty came through glaze....but generally others say glaze higher!)

Form is more important to me than the decorative effects. I LOVE my unglazed, bisqued, smooth white earthenware, and am exploring subtle oxide additions for some pieces. My best pieces recently, were clear glazed on the inside and unglazed on the outside - no 'pinking' on these pieces, but 2 that were glazed inside and out now have long cracks in the glaze. It's so frustrating when so much work has gone into the pieces - throwing, carving, incising, building on etc. then a successful bisque, all to be lost in the glaze firing! ANY ADVICE/SUGGESTIONS would be most welcome.


I know it may seem very straight forward, but since you say you mix your glazes from powder, are you using the same Kaolin (clay component) that your clay body is made from? Often times there are very subtle differences in the clay that is pulled from one location and the clay from another location that can actually affect fit. I've always made sure that even if a glaze recipe calls for a different kaolin to still use the same kaolin that is in my clay for fit issues. So far, cross my fingers, I have not had any colorant issues from doing so.

I also have had issues with bubbles & crazing in my clear glazes and have found a wonderful resource in The Potter's Dictionary by Frank and Janet Hamer. It is a comprehensive resource for almost all things clay. Including what goes wrong, and possible ways to fix it. I found that adding a bit of frit which contains borax can really help improve the clarity and fit of the glaze. The frit I used is ferro frit 3195, a high calcium borate frit from my local supplier, in a concentration of 2-5% in the glaze.

Obviously, more testing will need to be involved to determine what will work with your specific resources and environment. I use a numbering system on my test tiles and use that to test one variable at a time (the only thing being hard to test specifically is the firing schedule/temp). That way I can cover a bunch of different variables in one kiln firing and adjust based on the out come.

I hope this helps as one emerging artist to another.

Shannon EW Schanus

#6 Celia UK

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Posted 22 December 2012 - 12:16 PM


I am fairly new to ceramics, very new to firing my own work. Have small 40 litre electric kiln - make small thrown pieces. I mostly use smooth white earthenware and clear glaze with oxides added or over underglaze colours. I have fired various clays at the same time e.g. St Thomas white, and different White Earthenwares, I mix the glaze from powder and sometimes add oxides. I DO record details of all firings but the outcomes have been so inconsistent I haven't been able to identify a) ideal bisque firing temp and Posted Image glaze firing temp. Some pieces made of same clay body and with same glaze, are not equally successful. I have had 'pinging' problem with glaze cracking up to a week after firing on some pieces, (I understand this is a glaze fit issue), but I don't understand why this happens on some pots and not others? Everything is always bone dry before bisque firing and I leave the kiln until it is cool before opening. Have bisqued at 06 and glazed at 05: bisqued at 04 and glazed at 05 (read somewhere to bisque higher was better to ensure nothing nasty came through glaze....but generally others say glaze higher!)

Form is more important to me than the decorative effects. I LOVE my unglazed, bisqued, smooth white earthenware, and am exploring subtle oxide additions for some pieces. My best pieces recently, were clear glazed on the inside and unglazed on the outside - no 'pinking' on these pieces, but 2 that were glazed inside and out now have long cracks in the glaze. It's so frustrating when so much work has gone into the pieces - throwing, carving, incising, building on etc. then a successful bisque, all to be lost in the glaze firing! ANY ADVICE/SUGGESTIONS would be most welcome.


I know it may seem very straight forward, but since you say you mix your glazes from powder, are you using the same Kaolin (clay component) that your clay body is made from? Often times there are very subtle differences in the clay that is pulled from one location and the clay from another location that can actually affect fit. I've always made sure that even if a glaze recipe calls for a different kaolin to still use the same kaolin that is in my clay for fit issues. So far, cross my fingers, I have not had any colorant issues from doing so.

I also have had issues with bubbles & crazing in my clear glazes and have found a wonderful resource in The Potter's Dictionary by Frank and Janet Hamer. It is a comprehensive resource for almost all things clay. Including what goes wrong, and possible ways to fix it. I found that adding a bit of frit which contains borax can really help improve the clarity and fit of the glaze. The frit I used is ferro frit 3195, a high calcium borate frit from my local supplier, in a concentration of 2-5% in the glaze.

Obviously, more testing will need to be involved to determine what will work with your specific resources and environment. I use a numbering system on my test tiles and use that to test one variable at a time (the only thing being hard to test specifically is the firing schedule/temp). That way I can cover a bunch of different variables in one kiln firing and adjust based on the out come.

I hope this helps as one emerging artist to another.

Shannon EW Schanus



#7 Celia UK

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Posted 22 December 2012 - 12:27 PM

Thanks Shannon - though I think I misled you by saying I mixed my glaze from powder, as this is a prepared glaze powder from my supplier, rather than me mixing up from a recipe (too many ingredients to buy at this stage). Do you think I could still try adding the ferro frit? I'll have a look in my supplier's catalogue to see if it's available.

Latest firing was more successful. A few more things to learn from it - mostly in respect of my glazing technique. I need to improve my dipping skills and use of glaze tongs. Also as much of my work has holes pierced or incised there has to be a good way of getting an even glaze - with dipping, the glaze pours through the holes and it's hard not to get drips and runs. Do you think thinning the glaze somewhat would help? Alternatively, as I wondered in my previous post, perhaps bisquing at a higher temperature, making the clay less porous, would help. Oh the joys and frustrations.......

Celia

#8 SEWSart

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Posted 22 December 2012 - 04:39 PM

Thanks Shannon - though I think I misled you by saying I mixed my glaze from powder, as this is a prepared glaze powder from my supplier, rather than me mixing up from a recipe (too many ingredients to buy at this stage). Do you think I could still try adding the ferro frit? I'll have a look in my supplier's catalogue to see if it's available.

Latest firing was more successful. A few more things to learn from it - mostly in respect of my glazing technique. I need to improve my dipping skills and use of glaze tongs. Also as much of my work has holes pierced or incised there has to be a good way of getting an even glaze - with dipping, the glaze pours through the holes and it's hard not to get drips and runs. Do you think thinning the glaze somewhat would help? Alternatively, as I wondered in my previous post, perhaps bisquing at a higher temperature, making the clay less porous, would help. Oh the joys and frustrations.......

Celia


Ah yes, that may be a bit trickier to control since you would not be able to isolate a single component of the prepared powder, but it couldn't hurt to do a few 100gram test batches with different concentrations (say .5%, 1%, and 3%) of a frit that contains borate added to your glaze powder. It is a good thing that you get the glaze in powdered form, because if it were premixed with water then it would be much harder to consistently add the same amount of frit to each glaze batch (if the testing turns out).

I have definitely found that crazing will happen when there is too thick of a glaze application. Using just clear glaze really doesn't take much to make a nice glossy seal. If you take a credit card and scrape a line in the glaze you should see that the thickness on the piece is about the width of credit card or even a touch less (you can then rub your finger back over the scrape to fill the glaze back in). Also, if you thin it down it would definitely help glazing the piercings. (I too am very much into pierced designs) I find if the hole has filled in as I pull the piece up from the dip, if I blow gently into the opening it will break the surface tension and possibly clear it. Otherwise if it stays closed, depending on the shape, i can use a small drill bit to hand "drill" the hole out without completely chipping away the dried glaze. You can take your finger and gently rub over the runs to "sand" them away so to speak, or you can use actual sand paper to gently grind them down. Just make sure to try to keep the dust down (wear a mask).

I was taught that when you stick your hand in the glaze and pull it out there should only be a thin layer still stuck to your fingers, if it's thick and sticks like gravy to the back of a spoon it's too thick. There is also a way to make a density meter with a block of wood and a weight so that you know the glaze is the same thickness every time you make it.
Basically you attach the weight to the end of the stick and place it in a bucket of plain water so that part of the stick is floating above the surface. Mark where the water level is on the stick.
Next, place it in the bucket of well mixed glaze that you know has been combined with the right amount of water (you've tested how well it covers the bisque and possibly fired to see if it crazes) and mark where the glaze level falls on the stick (it should be lower than the water mark).
Then, the next time you mix that glaze you can drop the stick in the bucket in between intervals of adding water to the powder until the stick comes to rest at the glaze level line. You should end up having the same density glaze every time. As with nearly everything, this was not my original idea. I believe I got it out of a Ceramics Monthly magazine actually. Posted Image

As far as firing the bisque to a higher temp then glaze firing, I've honestly never done so since I fire to cone 9/10 and the clay would never accept the glaze at that point since it would be completely vitrified. But I am curious if it helps with crazing at lower temps since it is much more economically feasible to fire lower.

Best

Shannon

#9 JBaymore

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Posted 22 December 2012 - 06:11 PM

Crazing happens because there is a difference in the (reversible) Coefficient of Thermal Expansion (COE) of the glaze layer and the clay body. Once the liquid glaze "freezes", as the kiln cools down from that point, the amount it shrinks is MORE than the amount the clay body shrinks. Glass is waek in tension and it releives the stress by cracking.... which we call crazing.

If a glaze crazes when thick, then it does not fit the body and will eventually exhibit delayed crazing in use even in the thin areas. You won;t see it (unless you do testing of the fired wares) but the customers/end users will.

The way to fix crazing is normally to recalculate (molecular glaze calc) the COE of the glaze to reduce the COE number. It usually is ratehr simple... one of the easier things to correct in many cases. But if it is a pre-manufactured glaze,...... you can't do that. The secondary method to fix crazing to alter the clay BODY recipe to cause it to have a higher COE. For earthenware, this is often done by adding in some ground cristabolite. Cristabolite has a high COE in the lower temperature ranges and dramatically increases the COE of a body that contains much of it (it causes significant dunting issues for stoneware potters firing long-cylce wood kilns).

If you can't alter the body either...... well you are going to have to do a lit of experimenting.

The standard trick for decreasing crazing is to add increments of SiO2 into the glaze melt until you hit the correct COE numbers. IN your case adding in very small additions of SiO2 by adding measured amounts of flint (quartz) to the glaze might do the trick. However as the silica content (SiO2) comes up... so does the melting popint of the glaze. SO hopefully you can add enough to fix the crazing without changing the appearance of the glaze.

To balance the effect of increasing silica on the cone end point, you can also add in a source of boric oxide. Boric oxide (B2O3) is a glass former like silica.... but has a much lower melting point. So you might be able to bring in a little boron to stave off the temperature raising impacts of the flint. There is no non-soluble raw materil that supplies ONLY boron in the melt... so this will start getting complicated. Boron also adds some elasticity to the boro-silicate glass that forms.... and that can sometimes help with the crazing tendencies also.

This will be very empirical testing and given your constraints a bit of a "shot in the dark" .... so prepare for a bunch of tests.

best,

.....................john
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#10 Celia UK

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Posted 23 December 2012 - 04:42 AM

Crazing happens because there is a difference in the (reversible) Coefficient of Thermal Expansion (COE) of the glaze layer and the clay body. Once the liquid glaze "freezes", as the kiln cools down from that point, the amount it shrinks is MORE than the amount the clay body shrinks. Glass is waek in tension and it releives the stress by cracking.... which we call crazing.

If a glaze crazes when thick, then it does not fit the body and will eventually exhibit delayed crazing in use even in the thin areas. You won;t see it (unless you do testing of the fired wares) but the customers/end users will.

The way to fix crazing is normally to recalculate (molecular glaze calc) the COE of the glaze to reduce the COE number. It usually is ratehr simple... one of the easier things to correct in many cases. But if it is a pre-manufactured glaze,...... you can't do that. The secondary method to fix crazing to alter the clay BODY recipe to cause it to have a higher COE. For earthenware, this is often done by adding in some ground cristabolite. Cristabolite has a high COE in the lower temperature ranges and dramatically increases the COE of a body that contains much of it (it causes significant dunting issues for stoneware potters firing long-cylce wood kilns).

If you can't alter the body either...... well you are going to have to do a lit of experimenting.

The standard trick for decreasing crazing is to add increments of SiO2 into the glaze melt until you hit the correct COE numbers. IN your case adding in very small additions of SiO2 by adding measured amounts of flint (quartz) to the glaze might do the trick. However as the silica content (SiO2) comes up... so does the melting popint of the glaze. SO hopefully you can add enough to fix the crazing without changing the appearance of the glaze.

To balance the effect of increasing silica on the cone end point, you can also add in a source of boric oxide. Boric oxide (B2O3) is a glass former like silica.... but has a much lower melting point. So you might be able to bring in a little boron to stave off the temperature raising impacts of the flint. There is no non-soluble raw materil that supplies ONLY boron in the melt... so this will start getting complicated. Boron also adds some elasticity to the boro-silicate glass that forms.... and that can sometimes help with the crazing tendencies also.

This will be very empirical testing and given your constraints a bit of a "shot in the dark" .... so prepare for a bunch of tests.

best,

.....................john




Thanks John - a very clear explanation and some specific things or me to work on. I can see some disciplined testing coming on in the New Year to reduce all the guesswork. The reason I'm going to these lengths, is so as not to waste two 5kg bags of glaze powder that I found at the back of the art store at the school where I used to be the headteacher (recently retired hence new interest in ceramics). I could just dump them and start over, but that seems wasteful & I could have the same problems with another glaze. Anyhow, it's a good exercise to go through and will be increasing my knowledge along the way. I need to be philosophical about ending up with unsuccessful test pieces and using up my clay stock, but I can also look on it as throwing practice, if I use small bowls rather than test tiles! I SHOULD be wrapping Xmas presents and doing the last food shop today, but I'm going to source the boric oxide and SiO2 (need to look this up!) online first!

Thank you so much for taking the time to reply - what a great way to get help, I love it!

Hope you have an enjoyable Christmastime.

Celia

#11 Celia UK

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Posted 23 December 2012 - 04:43 AM


Thanks Shannon - though I think I misled you by saying I mixed my glaze from powder, as this is a prepared glaze powder from my supplier, rather than me mixing up from a recipe (too many ingredients to buy at this stage). Do you think I could still try adding the ferro frit? I'll have a look in my supplier's catalogue to see if it's available.

Latest firing was more successful. A few more things to learn from it - mostly in respect of my glazing technique. I need to improve my dipping skills and use of glaze tongs. Also as much of my work has holes pierced or incised there has to be a good way of getting an even glaze - with dipping, the glaze pours through the holes and it's hard not to get drips and runs. Do you think thinning the glaze somewhat would help? Alternatively, as I wondered in my previous post, perhaps bisquing at a higher temperature, making the clay less porous, would help. Oh the joys and frustrations.......

Celia


Ah yes, that may be a bit trickier to control since you would not be able to isolate a single component of the prepared powder, but it couldn't hurt to do a few 100gram test batches with different concentrations (say .5%, 1%, and 3%) of a frit that contains borate added to your glaze powder. It is a good thing that you get the glaze in powdered form, because if it were premixed with water then it would be much harder to consistently add the same amount of frit to each glaze batch (if the testing turns out).

I have definitely found that crazing will happen when there is too thick of a glaze application. Using just clear glaze really doesn't take much to make a nice glossy seal. If you take a credit card and scrape a line in the glaze you should see that the thickness on the piece is about the width of credit card or even a touch less (you can then rub your finger back over the scrape to fill the glaze back in). Also, if you thin it down it would definitely help glazing the piercings. (I too am very much into pierced designs) I find if the hole has filled in as I pull the piece up from the dip, if I blow gently into the opening it will break the surface tension and possibly clear it. Otherwise if it stays closed, depending on the shape, i can use a small drill bit to hand "drill" the hole out without completely chipping away the dried glaze. You can take your finger and gently rub over the runs to "sand" them away so to speak, or you can use actual sand paper to gently grind them down. Just make sure to try to keep the dust down (wear a mask).

I was taught that when you stick your hand in the glaze and pull it out there should only be a thin layer still stuck to your fingers, if it's thick and sticks like gravy to the back of a spoon it's too thick. There is also a way to make a density meter with a block of wood and a weight so that you know the glaze is the same thickness every time you make it.
Basically you attach the weight to the end of the stick and place it in a bucket of plain water so that part of the stick is floating above the surface. Mark where the water level is on the stick.
Next, place it in the bucket of well mixed glaze that you know has been combined with the right amount of water (you've tested how well it covers the bisque and possibly fired to see if it crazes) and mark where the glaze level falls on the stick (it should be lower than the water mark).
Then, the next time you mix that glaze you can drop the stick in the bucket in between intervals of adding water to the powder until the stick comes to rest at the glaze level line. You should end up having the same density glaze every time. As with nearly everything, this was not my original idea. I believe I got it out of a Ceramics Monthly magazine actually. Posted Image

As far as firing the bisque to a higher temp then glaze firing, I've honestly never done so since I fire to cone 9/10 and the clay would never accept the glaze at that point since it would be completely vitrified. But I am curious if it helps with crazing at lower temps since it is much more economically feasible to fire lower.

Best

Shannon



#12 Celia UK

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Posted 23 December 2012 - 04:55 AM



Thanks Shannon - though I think I misled you by saying I mixed my glaze from powder, as this is a prepared glaze powder from my supplier, rather than me mixing up from a recipe (too many ingredients to buy at this stage). Do you think I could still try adding the ferro frit? I'll have a look in my supplier's catalogue to see if it's available.

Latest firing was more successful. A few more things to learn from it - mostly in respect of my glazing technique. I need to improve my dipping skills and use of glaze tongs. Also as much of my work has holes pierced or incised there has to be a good way of getting an even glaze - with dipping, the glaze pours through the holes and it's hard not to get drips and runs. Do you think thinning the glaze somewhat would help? Alternatively, as I wondered in my previous post, perhaps bisquing at a higher temperature, making the clay less porous, would help. Oh the joys and frustrations.......

Celia


Ah yes, that may be a bit trickier to control since you would not be able to isolate a single component of the prepared powder, but it couldn't hurt to do a few 100gram test batches with different concentrations (say .5%, 1%, and 3%) of a frit that contains borate added to your glaze powder. It is a good thing that you get the glaze in powdered form, because if it were premixed with water then it would be much harder to consistently add the same amount of frit to each glaze batch (if the testing turns out).

I have definitely found that crazing will happen when there is too thick of a glaze application. Using just clear glaze really doesn't take much to make a nice glossy seal. If you take a credit card and scrape a line in the glaze you should see that the thickness on the piece is about the width of credit card or even a touch less (you can then rub your finger back over the scrape to fill the glaze back in). Also, if you thin it down it would definitely help glazing the piercings. (I too am very much into pierced designs) I find if the hole has filled in as I pull the piece up from the dip, if I blow gently into the opening it will break the surface tension and possibly clear it. Otherwise if it stays closed, depending on the shape, i can use a small drill bit to hand "drill" the hole out without completely chipping away the dried glaze. You can take your finger and gently rub over the runs to "sand" them away so to speak, or you can use actual sand paper to gently grind them down. Just make sure to try to keep the dust down (wear a mask).

I was taught that when you stick your hand in the glaze and pull it out there should only be a thin layer still stuck to your fingers, if it's thick and sticks like gravy to the back of a spoon it's too thick. There is also a way to make a density meter with a block of wood and a weight so that you know the glaze is the same thickness every time you make it.
Basically you attach the weight to the end of the stick and place it in a bucket of plain water so that part of the stick is floating above the surface. Mark where the water level is on the stick.
Next, place it in the bucket of well mixed glaze that you know has been combined with the right amount of water (you've tested how well it covers the bisque and possibly fired to see if it crazes) and mark where the glaze level falls on the stick (it should be lower than the water mark).
Then, the next time you mix that glaze you can drop the stick in the bucket in between intervals of adding water to the powder until the stick comes to rest at the glaze level line. You should end up having the same density glaze every time. As with nearly everything, this was not my original idea. I believe I got it out of a Ceramics Monthly magazine actually. Posted Image

As far as firing the bisque to a higher temp then glaze firing, I've honestly never done so since I fire to cone 9/10 and the clay would never accept the glaze at that point since it would be completely vitrified. But I am curious if it helps with crazing at lower temps since it is much more economically feasible to fire lower.

Best

Shannon


Hi Shannon - thanks again! I've also read about dipping your hand in to test the glaze thickness, but I've found ideas of 'thin cream, thick cream, gravy, breaking over knuckles' etc a bit on the subjective side and dependent on my cookery knowledge / skills ! I did look at a hydrometer in the homeware store recently - having read that this was the 'proper' way to measure viscosity. I resisted buying one, but as it was less than £5 I may invest - though your idea sounds very clever too! Whatever, I need to identify the ideal measure first!!! Testing, testing....

Have a wonderful Christmastime.

Celia

#13 smastca

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Posted 24 December 2012 - 11:25 AM

As a relative newby to the whole world of pottery and glazing, I'd like to ask a technical question related to the two bags of glaze powder. Posted Image
Would the contents of the glaze settle out overtime? Would you then need to either remix the powder to use a portion of it or would it be best to make up the glaze with the whole bag at once?

Just curious and not meant to hijack the topic.
Susan






#14 bciskepottery

bciskepottery

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Posted 24 December 2012 - 09:09 PM

As a relative newby to the whole world of pottery and glazing, I'd like to ask a technical question related to the two bags of glaze powder. Posted Image
Would the contents of the glaze settle out overtime? Would you then need to either remix the powder to use a portion of it or would it be best to make up the glaze with the whole bag at once?

Just curious and not meant to hijack the topic.
Susan






In a dry state, the contents of the glaze would not separate. Commercial manufacturers mix dry glazes thoroughly so if someone buys, for example, 10 lbs but chooses to mix it 5 lbs at a time, there is no difference in color, etc. When you mix the dry glaze, you will add water and then sieve the mixture . . . sieving further ensures the glazes materials are mixed thoroughly. If a wet glaze mixture sets for a period of time, you may want to re-sieve it to make sure things are thoroughly mixed and blended. A wet mixture could settle.

#15 Celia UK

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Posted 22 November 2013 - 02:28 PM

John - almost a year after I started this one, can you believe I'm still having problems? To be fair, I'm only a hobby potter, with quite a few other retirement distractions, so am not working on my pots all the time! Anyhow, I recently decided to forget the 'old' bag of mid fire transparent glaze and buy a transparent earthenware glaze from the supplier of my clay. I thought all was well (have only tried a couple of pieces in 1 firing) but I looked at a piece today and again there were cracks (NOT CRAZING - but long, linear cracks, around the bowl). As this is the same fault as I had with the same clay, but different glaze I'm wondering if it's more to do with my bisque fire? I only have a small manual, electric kiln, so can't soak, but can ramp manually (dial runs from ON, through 1,2,3,4 to FULL). It has a kiln sitter and I've been using Orton minibars (06, 05, 04)but not cones or a pyrometer. I do start the bisque fire slowly - various speeds/ rates, but typically, say No1 for 1-2 hours no 3 for an hour, then FULL. 4-5 hours usually sees it finished, which has always seemed rather quick to me, but it's only a small kiln. I have a few successful pieces, some which I only glazed on the inside, another both in and out and I don't know what was different about this, compared to other failed pieces. I haven't quite managed to record the bisque temps and schedules alongside the glaze firing, so I'm thinking there are different combinations among my pieces!
My White earthenware has range of 1060 oC - 1160 oC and the glaze 1040 oC - 1140 oC.
Is the cracking more likely to be due to over fired or under fired bisque? Or too fast?
To slow it down my latest bisque fire was no1 for 2 3/4 hrs, then No3 for 1 1/2 hrs by which time it was glowing light orange/ yellow, I then reduced it to No2 ( had read that slowing down the last bit was an option). The limit timer switched off the kiln after 7 hours but the minibar 05 had not bent. To me the pieces have bisque fired, but I would say it's 'soft' bisque, given the colour through the top vent around 900oC. I don't know what to do with these - bisque again? And certainly don't know what to try for the next firing. It will only be small test pieces, as I'm getting fed up with failures! HELP!!




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