Glaze Bubbles

[High Bridge Pottery]

I have been searching for a bubble free glaze on and off for many years. There's an old thread here -

 

A lot of the time I can't even understand what I am trying to say in my old posts (and I wrote them xD) but if you want some context it might be worth a read. I would like to thank everyone for being so patient with me over the years, slowly stumbling about in the dark trying to understand ceramics. I very much appreciate it  

For the "Too Long Didn't Read" the old thread tried -

• Removing as many ingredients with loss on ignition as possible.
• Glazing an already vitrified clay body.
• Using holds and drop holds.
• Changing fluxes, silica and alumina amounts.

In the end nothing really seemed to produce satisfactory results except reducing silica in the recipe. There seemed to be a link between silica joining the melt and bubbles being produced.

 

Recently I have found a few more papers on this subject and wanted to share my findings. The first was in Transactions of the American Ceramic Society Volume II, Blisters in Glaze - Stanley G. Burt. https://archive.org/details/transactionsofam02colu/page/138/mode/2up

Back then they seem to have a lot more issues with sulphates coming from the fuel but Burt seems to find the same thing with silica kicking out gasses dissolved into the glaze.

 

 

It seems to be confirming that the more silica you have in a glaze the less dissolved acids/gas it can hold which ends up coming out of solution. 

After a bit more searching I managed to find the Seger paper in question in The Collected Writing of Hermann Seger Volume II, The Defects of Glazes and Their Causes - IV The Operation of Sulphates in the Glaze. https://archive.org/details/gri_33125001394697/page/582/mode/2up

 

 

Again he comes to the conclusion that silica is the main offender for producing bubbles. In a previously bubble free glaze remelted onto an already fired body it will take up silica from the body and produce bubbles in the glaze.

Back in Burt's paper in the discussion at the end he talks about a few ideas to remedy the problem. One is to have a glaze higher in silica which is the opposite to what I have found. His reasoning seems to be that the glaze would have less action on the body so reducing any silica coming from the body into the melt.
His second idea is to fire the bisque in reduction to volatilise the acid removing it from the body before the glaze firing. 

 

 

Seger also seems to find reduction is one remedy to the problem but does it in the glaze firing. The Collected Writing of Hermann Seger Volume II, The Influence of Sulphuric Acid on Glazes and Bodies. https://archive.org/details/gri_33125001394697/page/644/mode/2up

 

Some interesting things to think about, especially trying to do a reduction bisque firing and seeing if that produces any different results in the glaze. I am not sure I agree with Burt that a glaze higher in silica will produce less bubbles but maybe that combined with a reduction firing could produce results. Now I just need come up with some tests and see what happens.

Edited February 14, 2023 by High Bridge Pottery
Removed my text interpretation of bubbles from the title.

[Min]

Organics in the bisque fire not adequately fired off during bisque, lead glazes and reduction firing seems to be the gist of the quotes above, just wondering how much of this is relevant to the work you are doing now?

[High Bridge Pottery]

I don't think they are talking about organics in the bisque, most are burnt off pretty easily just the sulphur stays around. I feel it's pretty relevant to getting rid of glaze bubbles  the question is do I really have to get rid of bubbles you can only see through a microscope when the glaze layer is thin? Probably not but I want to see if I can.

I gave up trying to make money from my own pottery long ago and instead have fun testing and experimenting. I am not even sure how relevant my fritware tests were to any of my work but I enjoyed doing it. 

Edited February 14, 2023 by High Bridge Pottery

[Callie Beller Diesel]

I remember that thread! It was a pretty cool one. 

It’s interesting that Tony Hanson was pursuing a similar series of testing about the same time, seeking a really clear cone 6 fluid melt glaze. He kept going, doing a major update to his findings in 2019, and continues to add notes to the file. He took a slightly different tack than you, in that he was less focused on the silica beyond particle size as it contributed to melt, and wanted to embrace a multi flux approach for durability since his approach is based in making functional pottery. He did achieve a number of good results, and continued with the work to reduce COE values so it would fit a particular nuisance porcelain Plainsman makes. 

This is the link to phase 2 of his testing, which involved really eliminating bubbles and maintaining colour response. The links to phase 1 and 3 are at the top of the page.

I noticed in your original thread there were some mentions of using fining agents to get rid of seed in glass, and that you were using a red clay for some of your Currie blends. Iron is a fining agent in glaze, and if your aim is to create a clear glaze over a red clay, using an amber instead of a clear can be a solution. Reducing the high LOI ingredients and fine tuning your firing seems like the majority of the process, but if there’s remaining bubbles they might need that extra kick to evict them. 

[Min]

I read "Mr.Burt" as saying he thought this was a body problem more than a glaze problem. Also, if they are firing lead glazed ware we know that some forms of sulphur don't burn out until 700C - 1150C, which could mean much higher than lowfire lead glazed ware matures.

1 hour ago, High Bridge Pottery said:

the question is do I really have to get rid of bubbles you can only see through a microscope when the glaze layer is thin?

I wouldn't worry about glaze bubbles not seen by the naked eye, but could be a fun challenge to give yourself.

[High Bridge Pottery]

Thank you for the link, I have not seen those tests before so will have a look through his research. I am still unsure if fining agents will transfer across well to glazes but it is worth testing some out and seeing what happens. The red clay tests are actually Tim's tests and I am pretty sure he was using lead glazes. Never liked working with red clay myself.

 

He is saying it is a body problem but only related to sulphates as the rest of the organics will decompose happily at bisque temperatures, Neither of them bring up any temperatures and I think are just talking in general about the problem across a wide firing range. I am not sure all the sulphates do burn out even at that temperature. There's an interesting picture in the last thread where they measure the gas evolved from the batch containing 1% sodium sulphate and one is reduced by adding 0.2% carbon to the mix. In the oxidised batch they only start going out at 1000c due to some reaction and don't really decompose until over 1400c. I think I did read that sodium sulphate is one of the hardest do decompose.

It does make me wonder if I could even get them to decompose in a bisque firing with reduction, especially if I am only going to 800c.

 

 

Edited February 14, 2023 by High Bridge Pottery

[Bill Kielb]

9 hours ago, High Bridge Pottery said:

In the end nothing really seemed to produce satisfactory results except reducing silica in the recipe. There seemed to be a link between silica joining the melt and bubbles being produced.

This may be of interest to you. Not scientific nor entirely about bubbles but an interesting way to dial in best clarity. Just some studio tested methodology which focuses in on the perfect silica and alumina levels for best clarity. Anyway, other interesting aspects as Sue seeks some practical ways to obtain the clearest. https://suemcleodceramics.com/getting-clarity-with-clear-glazes/

[High Bridge Pottery]

It's a good place to start but I have never been satisfied with the answer to make the glaze thinner. I don't really see it as solving the problem just making it less visible. Not that Sue is claiming it does get rid of bubbles as she explains a thinner application just allows for a lower density of bubbles.

[Bill Kielb]

54 minutes ago, High Bridge Pottery said:

It's a good place to start but I have never been satisfied with the answer to make the glaze thinner.

I was thinking her optimum silica and alumina observation are consistent with your findings actually.  For clear glazes, thinner just a common sense pottery practice really.  Any thoughts of enhanced removal mechanically by ultrasonic excitation.

[High Bridge Pottery]

Looking at Sue's clear it is in the 7-8:1 Si:Al ratio that I have found to be best.

59 minutes ago, Bill Kielb said:

Any thoughts of enhanced removal mechanically by ultrasonic excitation.

 

I have certainly come across the idea but it seems very impractical to try and vibrate a kiln ultrasonically. I have also considered how I could add a potato to my glazes as they use that to remove bubbles in glass too. First time I read it I thought it must be a typo but turns out it is a valid technique.

 

 

[Rae Reich]

45 minutes ago, High Bridge Pottery said:

how I could add a potato to my glazes

Potato water after boiling potatoes? Dry Potato flakes?

Edited February 15, 2023 by Rae Reich

[High Bridge Pottery]

1 hour ago, Rae Reich said:

Potato water after boiling potatoes? Dry Potato flakes?

The issue would be it needs to be wet and put under the glaze when the glaze is molten to produce steam. Sorry it was a bad joke, I am not very good at jokes 

[Babs]

Don't joke befre you try it. Some folk spray starch to get the glaze to adhere properly :-000

Edited February 16, 2023 by Babs

[Bill Kielb]

On 2/15/2023 at 9:54 AM, High Bridge Pottery said:

The issue would be it needs to be wet and put under the glaze when the glaze is molten to produce steam. Sorry it was a bad joke, I am not very good at jokes 

Sounds like low power ultrasonic applied at the right time might be a mobilization and coalescing vehicle. Just need to make sure the kiln doesn’t turn into vintage vibrating football / soccer game.  I am thinking the mass of the ware is much greater than the molten glaze 

[High Bridge Pottery]

Having a look you can get some quite cheap ultrasonic generators, not sure that's the right terminology. It would be fun to try it but I can imagine turning it on and everything moving around and getting stuck together  best to try it first with a cold kiln and see what they do. Not sure how well it will transfer up the shelves and posts as they mostly seems designed for water baths.

 

Thinking about a way to do a reduction bisque. I had brought a propane bunsen burner with the idea of adding some gas into my electric kiln but I wanted to crunch some numbers to see how far I could get on the bunsen burner alone. Saves me taking the kiln outside and degrading the elements. I have 50ish insulating fire bricks sitting around waiting for me to do something with.

As far as I can measure the orifice is 0.75mm - 1mm somewhere in between that anyway. I have a 32mb/14.9wc propane regulator which says a max output of 1.5kg/h which seems to equate to 71,250btu/h.

Looking at orifice size I can get 7,680btu/h @ 14wc from 0.75mm and 13,900 btu/h from a 1mm orifice and I think it is sized somewhere between there. Olsen says that 10,000 btu/h is a good estimate for every 1 cubic foot of space.

Not wanting to spend loads more money I have tried to design an updraft kiln with a 9inch cube internal space, just under 0.5 cubic feet. This will let me build it without too many extra kiln shelves as hopefully the lid bricks will hold themselves together (I may have to add extra as weight to stop them falling into the kiln) and I can use normal bricks to hold the floor together and make enough space under the kiln for the burner. I have considered buying another bunsen burner and having two coming in horizontally at the sides instead of one in the middle underneath the kiln but this is where I am at.

 

Here's a quick design using 32 bricks.

Trying to decide if the flame will be fine going in straight and hitting the bottom shelf or if I should try to split the flame so it comes in on a 45.

 

Edited February 19, 2023 by High Bridge Pottery

[Bill Kielb]

14 hours ago, High Bridge Pottery said:

Having a look you can get some quite cheap ultrasonic generators, not sure that's the right terminology. It would be fun to try it but I can imagine turning it on and everything moving around and getting stuck together  best to try it first with a cold kiln and see what they do. Not sure how well it will transfer up the shelves and posts as they mostly seems designed for water baths.

Might be crazy but probably could figure out visually and confirm with strain gauges. It might be a big flop or wildly successful at removing bubbles or even influencing glaze surfaces, textures and the way a glaze breaks in a unique way…….. interesting!

Edited February 18, 2023 by Bill Kielb

[Rae Reich]

Piece of kiln shelf will reinforce the top. Can you make a little kiln shelf bagwall between the flame and the bottom shelf? Chimney, flue?

Edited February 19, 2023 by Rae Reich

[High Bridge Pottery]

A little bit of kiln shelf might do the trick between the flame and the bottom shelf. I was thinking about drilling/grooving the bricks so the entrance channel split into two at a 45.

 

I was hoping with the updraft I can just put a hole in the top and call it good. Not shown in the picture as I was trying to figure out how many bricks I will need and didn't go too far into modeling the whole thing. 

Edited February 19, 2023 by High Bridge Pottery

[Bill Kielb]

A well stocked drill index can make your orifice estimation easier. Reduction at bisque temperatures a bit difficult if I understand that correctly. We limit early reduction to 1500 f I believe to be safe. L&L has nice resources here https://hotkiln.com/sites/default/files/pdf/BTUS-Davinci.pdf for electric kilns and btu loss  per sq inch interior at various temperatures which can easily be translated to watts per Square (your area here) as a guide using 100% energy conversion.

Maximizing burner projection with minimal impingement maximizing the radiant area probably most important. I think I would have a good idea of the energy required at 100% efficiency and see what the burner projection or flame  length at approx 150% of anticipated energy needs ends up as a start.

Edited February 19, 2023 by Bill Kielb

[High Bridge Pottery]

Yea I am just eyeballing it with a ruler, not sure I have a drill bit under 1mm but I will double check.

 

What are you being safe in regards to? I will have to fire in oxidation and probably hold a little at 800c (1427f) to burn out all the organics then put into reduction to in theory burn out the sulphur. I could probably go to 900c (1652f) without it losing too much porosity but with reduction that may get it too vitrified.

 

Looking at that link I have 9x9x6 square inch so 486 and if I divide that by 144 (1 sq ft) I have 3.375 and multiply that by 526 btu loss for 1175.25 btu/h lost through the walls. The side walls are using the 4.5 inch width but let's call it 3 anyway.

You have lost me at the end, I want to maximise the length of the flame with minimal things in its way?

[Bill Kielb]

3 hours ago, High Bridge Pottery said:

What are you being safe in regards to?

Natural gas / propane - auto ignition temperature is 482c - 649 c (1200 f) To be very safe so folks don’t create their own bomb we usually require the temperature of early (body) reduction to begin no less than 1500 f to be extra safe.  No point in finding the cool spot in the kiln to only to blow up the studio.  

The L&L docs are for interior square inches so surface area calc. The data show for 3” walls kilns are designed in the 3.3 - 4 or more  watts per interior surface area. Good data actually to gauge kilns by along with recorded losses at various temps.

If you need 30,000 btuh and that burners flame length at that output or above is 12”, then the flame will impinge on the wall significantly for any shape less than 12”  so to me, figure out required btuh, add your margins of safety and check burner requirements such as pressure, orifice, and flame length. Almost ALL of your heating comes from radiation, so an effective flame is key. Convection - not very significant especially at high temps.

Edited February 19, 2023 by Bill Kielb

[High Bridge Pottery]

Thank you for the explanation. Going to have to do some thinking about my burner placement and if two going in horizontal at the sides will be much better than 1 going in the bottom at the middle. I thought that's what you meant about safety but wanted to double check.

 

A 9inch cube with 10,000btu/h burner (2930 watt) gives 6.03 watts per internal square inch and 12inch cube give 3.4 watts per internal square inch so it looks like Olsens 10,000 per cubic foot is the upper limit of what's possible for a tiny kiln. I would guess it gets better for bigger kilns as less surface area to internal volume.

Edited February 20, 2023 by High Bridge Pottery

[Bill Kielb]

2 hours ago, High Bridge Pottery said:

A 9inch cube with 10,000btu/h burner (2930 watt) gives 6.03 watts per internal square inch and 12inch cube give 3.4 watts per internal square inch so it looks like Olsens 10,000 per cubic foot is the upper limit of what's possible for a tiny kiln. I would guess it gets better for bigger kilns as less surface area to internal volume.

The thing that might interest you would be for 3” brick at 2350 f losses through the kiln wall end up to be about 5.29 btu per sq inch or 1.55 watts per square inch and need 3.5 - 4 + watts per square inch to work effectively over a reasonable number of firings.

So pretty easy to know How many watts it would take to supply for your kiln given heating is prox. 100% efficient. Translate that to gas by understanding your burner will probably operate in the 70-80% range and the whole kiln thermally will probably in the 50% range. At least you have a credible idea of what your burner size might need to be.

Not sure all that helps but gives you credible idea of losses at 2350 f (cone 10)

Edited February 20, 2023 by Bill Kielb