Looking for Help! I glaze fired stoneware mugs to cone 6. The clear glaze is has come out rough instead of shiny. Worried they won't be food safe?

[Steph123]

Hello!

I'm pretty new to ceramics so looking for advice!  I have glaze fired stoneware mugs to cone 6. (1220 C) The clear glaze  has come out with a rough to touch texture instead of glossy/smooth. I applied 3 coats and it looks evenly distributed just that it feels rough.

The rest of the colours I used have turned out glossy.  I glazed the inside of the mugs with  'Vitraglaze Stoneware Glaze: Shiny Transparent' 

The mugs are for a friend and my main concern is that they won't be food safe? How do I stop this happening again?  Would appreciate any advice. 

Best, 

Steph

[Hulk]

Hi Steph!

While waiting on replies, could you post pics and identify the clay you used?

My guess - given the liner glaze is not crazed (cracked), and the layer is not interrupted, it should be ok, although a bit more difficult to clean.

From there, do you think the liner glaze is thinner than the outside/colored glaze layers? My observation, when (some) mid fire clays are a bit overfired, and weep tiny droplets, a thicker glaze layer incorporates them ok, where a thinner layer does not. These rough particles, on close examination (with magnification; my vision isn't great) - those on the unglazed foot ring - appear smooth, like a droplet, and can be scraped off.

Another theory you may consider: a sanded and/or grogged clay can feel rougher after glaze firing, as the "clay" portion shrinks, whilst the particles do not. Using an edge - typically, a rib - to smooth the clay can help, as the particles are driven in, and the "cream" coaxed to the top, rather like finishing concrete, where the gravel (or gravels, depending, eh?) are not discernable at the finished surface.

[Dick White]

The vitraglaze sales page indicates this is a cone 6 to 9 glaze. Are you certain you fired to cone 6 as shown by witness cones on the shelf of your firing, or just that you think it was 1220C which under the proper circumstances with an accurate thermocouple should have been cone 6. You are firing at the minimum specified temperature/cone, so any underfiring will come out rough like that. Also, interiors of mugs and closed forms  tend to be cooler than the exterior.

[Steph123]

1 hour ago, Hulk said:

Hi Steph!

While waiting on replies, could you post pics and identify the clay you used?

My guess - given the liner glaze is not crazed (cracked), and the layer is not interrupted, it should be ok, although a bit more difficult to clean.

From there, do you think the liner glaze is thinner than the outside/colored glaze layers? My observation, when (some) mid fire clays are a bit overfired, and weep tiny droplets, a thicker glaze layer incorporates them ok, where a thinner layer does not. These rough particles, on close examination (with magnification; my vision isn't great) - those on the unglazed foot ring - appear smooth, like a droplet, and can be scraped off.

Another theory you may consider: a sanded and/or grogged clay can feel rougher after glaze firing, as the "clay" portion shrinks, whilst the particles do not. Using an edge - typically, a rib - to smooth the clay can help, as the particles are driven in, and the "cream" coaxed to the top, rather like finishing concrete, where the gravel (or gravels, depending, eh?) are not discernable at the finished surface.

Hi Hulk. Thanks for your response. I used  this clay:

https://www.hot-clay.com/vitraclay-premium-white-throwing-clay-high-fire.html

During application the clear glaze did feel thinner in comparison to the coloured glazes. I have attached some photos to show the results of the clear glaze post firing in comparison to the exterior cobalt blue. 

So my understanding is that this should be okay for drinking from. Just to pay closer attention to cleaning it? And for next time maybe more applications of the clear glaze needed.

 

[Steph123]

1 hour ago, Dick White said:

The vitraglaze sales page indicates this is a cone 6 to 9 glaze. Are you certain you fired to cone 6 as shown by witness cones on the shelf of your firing, or just that you think it was 1220C which under the proper circumstances with an accurate thermocouple should have been cone 6. You are firing at the minimum specified temperature/cone, so any underfiring will come out rough like that. Also, interiors of mugs and closed forms  tend to be cooler than the exterior.

Hi Dick, 

Thanks for your reply.

As I am new to ceramics I'm currently using a kiln service. They assured me that the temperature was reached. Wondering if it perhaps was under fired as I asked for 1200 which is the minimum temperature specified for the glaze. 

[oldlady]

steph, that is a "learning from" or practice mug.   please do not give it to anyone.   save it and learn from what you have done.  match the glaze to the clay and use only materials that are the same cone number or temperature number.   wide swings of temperature are to be avoided.

in addition, try using that mug handle.  i will bet that you find it uncomfortable as it is.  the idea is Ok but your execution might be improved by making the hole bigger and leaving a space under the hole for the rest of your fingers to support the weight.   if you had planned to give it for christmas, give it with a note that when you are more skilled your friend will get a better one and take it home with you so your friend does not try to use it.

[Dick White]

2 hours ago, Steph123 said:

Hi Dick, 

Thanks for your reply.

As I am new to ceramics I'm currently using a kiln service. They assured me that the temperature was reached. Wondering if it perhaps was under fired as I asked for 1200 which is the minimum temperature specified for the glaze. 

Steph, we were all new to ceramics at one point and we are all still learning something new every day. Regarding the concept of cones and temperature, they are not the same. Temperature is a measurement of an absolute thermal condition at a single point in time. Cones measure heatwork, which is pottery-talk for heat penetration into the core of the ceramic body to achieve proper vitrification and bond between the glaze and ceramic. It can be quantified by the heating rate (time and temperature increase) over the last 100℃ of the firing. If the kiln was firing at a medium rate of increase (60℃/hr.), 1200℃ would have been cone 5 ½. If the heating rate was much slower, (60℃/hr.), then the same absolute 1200℃ would have been a cone 7. If the kiln was firing fast at a rate of 150℃/hr., then it would have been a soft cone 5. So unless you know the heating rate of the kiln at the end (and many kilns actually slow down below the expected rate of the program towards the end), the pure temperature may or may not indicate the expected cone was reached. In this case, because you were operating at the extreme lower end of that glaze's maturation range, it could be underfired.

Oldlady gives some very good advice above, both about handle design issues and a graceful way to handle relations with your friend.

[Steph123]

2 hours ago, oldlady said:

steph, that is a "learning from" or practice mug.   please do not give it to anyone.   save it and learn from what you have done.  match the glaze to the clay and use only materials that are the same cone number or temperature number.   wide swings of temperature are to be avoided.

in addition, try using that mug handle.  i will bet that you find it uncomfortable as it is.  the idea is Ok but your execution might be improved by making the hole bigger and leaving a space under the hole for the rest of your fingers to support the weight.   if you had planned to give it for christmas, give it with a note that when you are more skilled your friend will get a better one and take it home with you so your friend does not try to use it.

Oldlady, 

Thank you for your advice. I am glad I reached out on the ceramics community here! Lots of lessons to be learned. Ceramics is a very rewarding experience so I would love to persevere. If you have any reading recommendations I would be happy to hear about them!

Best wishes 

[Steph123]

30 minutes ago, Dick White said:

Steph, we were all new to ceramics at one point and we are all still learning something new every day. Regarding the concept of cones and temperature, they are not the same. Temperature is a measurement of an absolute thermal condition at a single point in time. Cones measure heatwork, which is pottery-talk for heat penetration into the core of the ceramic body to achieve proper vitrification and bond between the glaze and ceramic. It can be quantified by the heating rate (time and temperature increase) over the last 100℃ of the firing. If the kiln was firing at a medium rate of increase (60℃/hr.), 1200℃ would have been cone 5 ½. If the heating rate was much slower, (60℃/hr.), then the same absolute 1200℃ would have been a cone 7. If the kiln was firing fast at a rate of 150℃/hr., then it would have been a soft cone 5. So unless you know the heating rate of the kiln at the end (and many kilns actually slow down below the expected rate of the program towards the end), the pure temperature may or may not indicate the expected cone was reached. In this case, because you were operating at the extreme lower end of that glaze's maturation range, it could be underfired.

Oldlady gives some very good advice above, both about handle design issues and a graceful way to handle relations with your friend.

Thank you so much! It's hard not to feel disheartened but lots of lessons to learn.

Could recommend any reading materials? 

[Callie Beller Diesel]

Do you prefer hard copy, or websites? There's loads of both. 

 

[Steph123]

10 minutes ago, Callie Beller Diesel said:

Do you prefer hard copy, or websites? There's loads of both. 

 

Hi Callie,

Would appreciate both! like to mix it up so not spending too much time on screen. 

[Hulk]

Hi Steph,

I can't quite tell from the pictures if the roughness is due to bubbles or what - is the liner smoother where thicker?

Any road, lots to consider from the several who chimed in!

There are threads here listing resources; here's two

   Tony Hansen's website - articles, glossary, recipes, a treasure, indeed https://digitalfire.com

    Peterson's The Craft and Art of Clay (a book); I like how she explains unity.

[Steph123]

11 hours ago, Hulk said:

Hi Steph,

I can't quite tell from the pictures if the roughness is due to bubbles or what - is the liner smoother where thicker?

Any road, lots to consider from the several who chimed in!

There are threads here listing resources; here's two

   Tony Hansen's website - articles, glossary, recipes, a treasure, indeed https://digitalfire.com

    Peterson's The Craft and Art of Clay (a book); I like how she explains unity.

Hi Hulk,

After closer inspection under a magnifying glass it appears to be tiny bubbles. It is smoother where thicker too!

 

Thanks for the recommendations. Great help!

[Hulk]

I definitely need a magnifying glass, quite often, heh, also excellent lighting, and then I may see, given that some time has passed since unloading the kiln...

Some bubbles will have to be ok - given one looks closely enough, bubbles will be found - but an obvious froth may be too much. Where come the bubbles, aaah, clay or glaze? Yep. My guess is the clay, where your blue clears the bubbles better - easily tested, put some blue onna inside of a test case, with an identical (err, similar) control case right next to it. If clay, then a glaze that behaves differently, and also maybe alter the firing. If glaze, then alter the recipe (or start over).

[Callie Beller Diesel]

It looks a bit underfired. 

[Chilly]

On 12/21/2020 at 10:07 PM, Steph123 said:

Could recommend any reading materials? 

Everything and anything you can get hold of.

Try library for any books on pottery.  Even if they're really old.  The more you read, the more you will absorb, and the more you will understand.  It takes a while for some of it to sink in, but keep going.

Read any links on this forum.

 

Also, can you please update your profile info.  We're guessing at some answers which might be different depending on where in the world you are.  As you used 1200c as a temperature, I'd guess you are in the UK, but others might think you are in Canada or Australia.

[PeterH]

You may find something of interest in: Pete Pinnell: Thoughts on Cups
https://youtu.be/WChFMMzLHVs

 

 

[Sorcery]

I quite like your "half a butter cookie" handle, even if it ain't comfortable! Lol! Some mugs are made to hold and handles to look at!

Beware the bubbles, I had some on a piece I was rubbing the other day thinking, this wasn't so white before...It was cutting the dead skin off my thumb like a cheese grater, that was turning it whiter. Coulda been disastrous on tiny child fingers!

Sorce