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Ian Currie Test Tiles Forums?


curt
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On 20/11/2017 at 10:00 PM, Joseph F said:

I understand that most people(99%) would think it is way too much work to run tiles on every single glaze and clay body. I am talking about 105 vertical tiles and 3 grids each time I run a Currie test now(3 clay bodies). The issue is that we have the glaze for the tiles. It is sitting there for us in a cup ready to be dipped. Why don't we do it? I wish Currie was alive so I could ask him over email. I assume he probably kept it simple because like Neil and others have mentioned in this thread, a lot of the glazes produced by the grid method are mostly useless for dinnerware. But if we are only working for dinner ware, we would be excluding discovery of glazes for half of the ceramic field who do sculptural work and pieces not designed for food. 

I would happily run vertical tiles for every one of the 35 glazes on a standard Currie grid if it were not so much work.  I am sure it would lead to the discovery of a bunch of interesting glazes that I am missing now because I do not go this extra step.  Alas, it does take extra work - producing blank test tiles, firing them, etc.  but the biggest problem I really seem to having even selectively running vertical tests is that my standard 300g corner batches do not really generate enough glaze in the cups to properly dip a test tile later.  Also, since the delay between initially mixing up the  cups and later wanting to run the vertical tests is often a few weeks, I find I usually have to kind of reconstitute the glazes in each cup to get them the proper consistency.  More time, and then if I add a bit too much water, well, now it is too thin.  

I am thinking I could do a better job of using ALL the glaze in the corner batches into the cups, giving me more to work with in each cup, ie more to dip into with a vertical test tile.  Or maybe I could mix bigger corner batches in the first place and have more to work with.  Or maybe I could decant the glaze from each cup after mixing into a purpose-made, perfectly-fits-the-shape-of-one-vertical-tile kind of cup that maximises the glaze coverage on the test tile.  

I know that some here have gone the opposite direction by mixing the smallest possible corner batches so as not to waste raw glaze materials.... but I am probably headed the other way, as it is enough work and time to do a Currie grid that I am not worrying about wasting a few bucks on materials.  Anyway, still thinking about this.  One thing I HAVE done with some of the leftover corner batches and cups so as not to waste them is to remix subgroups of cups and corners to make a smallish batch of a whole new glaze which approximates the chemistry of one of the cells.  This has worked to some degree.

On 20/11/2017 at 10:00 PM, Joseph F said:

There is definitely a problem with running glaze test consisting of 100% flux. What is the point of it at all?

If you mean on vertical tiles I agree.  I think the biggest value of the 100% flux cell is when it is on a flat Currie tile for comparison purposes.  That 100% cell gives a clear idea of what the glaze looks like with NO added alumina and NO added silica, so we can get a sense of how the glaze changes when we start adding those materials (which make up the majority of most glazes).  More generally, I would say this is true of a number of the extreme or peripheral cells on a Currie tile.  They are not so much “glazes” as they are visual points of comparison - and thereby information- on the cells in the interior space that we are probably more interested in as viable candidates.

I am (trying) not (to be) a purist on Currie’s method.   I know we have collectively made improvements to it already and I agree with you that there is more to be done.

 Yes! We have to keep pushing it, testing it, rethinking it - and ultimately making it better.

 

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@curt

Well said!

I agree on the material waste. Not a big deal. 1200g of materials, mostly cheap flux, clay and silica are not a huge issue. At most I would assume a Currie test cost $6-7, and that is pushing it. I would say the average cost of a Currie grid test is around $3-4 in clay and glaze materials. The real issue is time. You spend a good hour and half doing the original tile, then if you do any additional mixes using the addition method I outlined you spend another 20 minutes for each additional mixing and such. So time is definitely the real issue here.

For mixing the cups and test tiles. I am going to try slanting the cup so that the cup has more depth for the tile to go down into. I am also going to dip the tiles first then do the syringes. Because I just need a tiny amount to do a syringe, but I need a large amount for the dips. So I figure I will mix the cup I am working on, dip 3 tiles. Then syringe out 6ml and put 2 ml on each grid tile for that number.  It will definitely add time to the overall process probably a good 30 minutes of additional time, but the grand plan is to see everything possible in one firing! 

If the slanted cup doesn't work, I will do what you are thinking and just increase the corner batches significantly so that the cups can have 60-70ml in them (500-600g corner batches). With the extra glazes in the cup from increasing the corner batches I can then default back to the incremental method I outlined. Because I can syringe out the entire cup and put back 40ml into the cup, squirt the rest into a waste bucket. Then I can have the ml needed to do incremental blending for SiC/colorants as each cup will contain the 40ml for easy blending. So even that will not be a waste. Of course now I am talking about 3-4 hours of work, and another issue of no verticle SiC tiles. lol.

If we all keep this discussion going and continue to post any ideas and discoveries I am sure we can continue to optimize the process. 

Happy Thanksgiving to you all.

Edited by Joseph F
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On 20/11/2017 at 4:18 AM, Joseph F said:

Now that I have a slab roller and I can roll a slab to the same thickness every time it means I can create uniform test tiles that are the same thickness as well. So I roll out a slab, put ruler over it and cut out a bunch of rectangles that are flat. Then I will just roll a slab of clay take a tool(made from the slab size I want) and make 7 trenches that are the same width as the tiles that I am using.

Something like this:

gridforvert.png.83b830763d8109e5edb230683e2262cc.png

Then when I do a currie test I will have 2 tiles side by side, and 35 test tiles that are just rectangles. I will mix the cup with milk frothier, syringe the glaze into the square, then dip the tile into the cup and then place it into the trench on the other tile in the same place as the grid. Thus if any glaze runs or what ever it will be contained. This is a really fast way for me to get the all the information in a single firing!

I think this is a good direction to follow. For my regular test tiles I've been using something similar, except that instead of trenches I just have one or two supports on which I can lean the test tile. The supports are in the shape of triangles pointing upwards, if that makes sense. I put a layer a kiln wash over the support, so minor glaze runs don't ruin it. It could do with some improvement though, since my prototypes are pretty crude, and sometimes the tiles stick together.

Anyway, for vertical tiles for a Currie grid, you could make 7 slabs, each with a row of 5 rectangle-shaped depressions, like the squares in a Currie grid, into which you could pour the individual glaze tests at the same time that you fill the corresponding squares in the regular grid. This way it'll be easier to keep the tests in the same order as the regular grid, and you won't have to dip them. A potential problem is that the very runny glazes will have a puddle of glaze attached to the bottom, assuming you can remove them from the support, which will make stacking them for storage difficult. But you could just omit the ones around corner C.

Actually, it might be better to have 5 slabs with 7 depressions each, otherwise when you put the slabs together flat, the height of the whole thing will be much greater than the width.

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It is all so complicated when you factor in storage. I am not sure how well the trench plan will work. I will know more next week after I run a grid. Currently I am to busy with term projects to fool with anything.

I am not sure I am too worried about the storage of tiles long term.  I am assuming the C tiles will be mostly useless as vertical observations, but I don't want to jump to conclusions quickly. I was mostly thinking about documentation of the vertical tiles by picture, description, and surface type. Then trashing them. Otherwise I think I might get overrun with tiles pretty fast. I am going to be firing 100+ tiles for each grid basically. 35x3 + the flat tiles.  I cant imagine keeping everything. There has to be some rigorous documentation somewhere. I was actually thinking about building some type of webapp for it or something. I love programming, but I only love it when I have an actual use that I need to resolve.

 This is probably going off topic, but I find that the way we test and record glaze data is really bad. Before and after glaze shots are super important for actual pieces along with the process of application. I usually do all this on paper, but a few months later I never can remember what went with what because there isn't a picture beside the description of the pot and if I kept every interesting test tile or pot I had made I would be out of room. I usually take a picture and label it.

I tried doing this inside of insight, but I didn't feel like it was intuitive enough. I want something that flows much easier and I get tired of how insight does a few things. So I basically quit using it for glaze documentation and more for glaze development. I think I have said this before, but what I want in an app is a simple process of application methods, pictures of before and after, surface qualities and color variations. 

For example you have a glaze your working on. You create a page or whatever for it.

On the page you have: Glaze Name, Description, Cone, etc .. basically all the ways you want to categorize it for searching through glazes.

Then you have pictures of the base and grid tiles in high resolution. Then you put up work that was glazed in this glaze, including before and after shots with your notes. This way you could look through and see how you glazed a pot to get that effect. 

Then I would like to also have an area of tiles with color variations of line blends and such. 

And finally any time you find a good modifier to go over it you document those tiles as well and they appear in both glaze topics.

So 2 years later you want a good matte base glaze that goes well with say: a fake ash glaze or something. You could search by tag: matte, fake ash and it would bring up tiles that had those tags together. Or you could just go to the fake ash section and look at tiles that had other bases that went well under it.

This way over a life time you could record everything really well. I highly doubt this would be marketable though as most people don't do enough testing to warrant paying money for it. But for my personal use I think it would be wonderful. It would be fun to build as it would be pretty simple to do. Basically a database and a bunch of queries for the pages and then just display it in a simple static page load no reason to get all fancy. 

Edited by Joseph F
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I'm going to have to start ditching my test tiles at some point too. I don't fire that often, but they're already staring to clutter up my studio. I'm just a bit wary of relying on photos, since there are some aspects of glazes that are hard to capture in a photo.

I use Glazy for recording most of my tests, but a system that naturally caters for recording things like application thickness and firing cycles would be ideal. I know that Derek Au, the creator of Glazy, has thought about this, but at the moment he's busy rewriting the current version, so I don't think it'll happen any time soon. The new version of Glazy will be open-source, though, so if you're up for doing it yourself, this would be a good place to start.

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More off topic here:

I agree that glazes can be very hard to photograph. Usually the more interesting the glaze the harder it is to photo correctly, so there is a major problem there. But with some good note taking and detailing it should be doable to a point. 

Glazy is awesome and Derek Au has a lot of great articles and free stuff he shares, the issue with everything that I see involving online recipes is that all the information they contain besides what cone it was fired to, is picture and recipe. There is never a schedule, an application process, what clay body, what glazes go good over it etc. So what ends up happening is you take a recipe and mix it up, then fire it and you get something completely different. So you just abandon it right away the majority of the time.

What we really need in the glaze testing world is a standard for sharing. Basically a uniform method of sharing a glaze recipe that includes more than just: here is a picture, cone6, recipe, and good luck.

However again, this is mostly unlikely as I think most potters who do the work for their own glazes probably don't want to share as there isn't much in it for them besides their work being duplicated which seems to be really popular. Glaze tiles are like humble brags in all honesty. Look at this! Then 10 people ask how you got it, and the author never responds. I have even done this if I am being honest here. Now I mostly refrain from posting anything interesting to avoid this whole conflict. 

Further, you get into the topic of most single glazes are pretty boring, the real interesting stuff comes from layering glazes. Which is an entire new combinational monster to document and test. 5 glazes is 5! (120) combinations of possibilities with just a single over layering. It can get daunting insanely fast. There is no perfect solution.

I would at least like some type of standard along the lines of the following information to submit a recipe:

  • Name
  • Firing Type (Soda, Wood, Salt, Electric, Gas, ETC)
  • Clay Body Name
  • Schedule: (Default Controller, Slow Cool)
  • Type of Surface: Matte, Satin, Glossy
  • Application Thickness: (1 layer, 2 layer, 3 layer)
  • Glaze Flow: (Runny, Normal, Stiff)

I could go into more detail for my own glazes, but I mean these 7 points are super easy to provide and could drastically improve the glaze recipe world. If you don't wanna submit this information why even share a recipe in general?

Ideally if I was building a software for sharing I would include some way to create schedules inside of your user profile, so that when you upload a tile you can just link one of the schedules you use with it. This would include the firing type as well to avoid extra process, and you could also have clay types that you work with. This way it would just be checking a few boxes or clicking a few drop downs, then typing the name, layers, and flow. Done. You should even be able to set defaults if your one of the type of people who shares often usually its going to be on x body, with y schedule and z firing type.

What is even more important is later when people test things over other things the linking of data between glazes is of utmost importance. Say you like John's Blue, and you then test a glaze over it called Red Purple or whatever, when you upload the red purple tile, you can upload a tile with it over johns blue and tag johns blue, so that later when someone else comes along and is looking at johns blue they can see the variations from other glazes and vice versa . We are seriously lacking this information. 

Edit: Removed potential breaking of rules.

If I am going to be pumping out thousands of glaze tiles and layering combination test and such, what am I going to do with all of it? Just because I don't like a tile and I trash it doesn't mean another person wouldn't like the glaze and all the variations and modifiers that go over it.  I cleaned out my garage and I had over 400 small containers of glaze test batches from 3 years of glaze testing. This isn't including all the line blends and stuff that I had done in solo cups that dried and got tossed into my waste bucket.

Anyways back to Currie discussion? Did you ever do more oilspot test with that cone 4 oilspot? I would love to see it on a pot. 

Edit: Upon reading this I realized the 400 containers sounded like a brag. I meant for it to show how horrendous I was at testing glazes in a good manner. I would just mix up 100g batch of a recipe and then if I wanted to test darkening it by 1% I would get another container, mix 100g again and test it. Never knew about all these volumetric and dry batch measuring processes.  So I am sure other people are doing the same horrible mistakes. 

Edited by Joseph F
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Well, on Glazy you can specify the firing type and type of surface, although not everyone does this. I agree that the other variables are important, and should be included if the goal is for other people to replicate your glaze. On the other hand, I think of recipes I find online as starting points for tests, since most of the time I have to (a) tweak them to work at cone 4 instead of 6, and (b) reformulate them in terms of the ingredients I have available. Also, I'm limited by how fast and how high I can fire, so even if someone gave a firing schedule for cone 4, I might not be able to follow it.

Back to Currie grids: I did another grid not that long ago which I'm really happy about. I'll post pictures once I have time to pull my thoughts together. In the same firing I tried out a couple of glazes from my first grid. The photos below are of the one in the middle of the bottom row (tile 33):

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The one on white stoneware went on thicker, and ran, as you can see. The one on red stoneware has fewer pinholes (not sure if that's due to thickness or claybody), but the spots are smaller. If there's one thing I've learnt from these tests, it's that I need to use more glaze and a bigger container to dip in. I had planned to dip in and out slowly to ensure that the top of the bud vases had a thicker application, but I was too busy trying to fit them into the container to do that.

The other oilspot type glazes I tried were less successful.

PS: Happy Thanksgiving, for those of you who celebrate it.

Edited by Pieter Mostert
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Here are the results of a Currie-type grid where I again varied iron and alumina. The proportions of everything else are fixed, and the same as my previous test, except that silica is now at 2.48 UMF (previously it was between 2.12 and 2.27). Based on the results of the first test, I reduced the range of iron slightly, to run from 0.09 to 0.22 UMF from left to right (previously 0.07 to 0.22). Alumina goes from  0.22 to 0.34 from bottom to top (previously 0.32 to 0.46)

Below is a screenshot from Glazy showing the corner glazes (with Fe2O3 not regarded as a flux). The ones circled in red are my first test, and the ones circled in blue are the second.

large.5a19427214ddb_HG7Stullchart.PNG.25319fe6123e1dcc0b8e40eec0373c00.PNG

I kept essentially the same firing cycle (slow firing to 1132C, with a 45 min hold at 950C on the way down. Cone 4 buckled but not flat on the shelf the tiles were on). However this time I fired the white stoneware above the red earthenware tile. This time I also made a little hole in the glaze in each square (based on Curt's suggestion of scoring a 6 o'clock line), but I can't see any evidence of them.

On red earthenware:

large.IMG_0024.JPG.c892ff8213f204330294a461f25b8a79.JPG

On white stoneware:

large.IMG_0020.JPG.2786be0b4af7b58c186bb41576019808.JPG


As in the first test, the tiles get progressively darker as you move from the bottom left to the top right corner.

Unlike the first test, which had unhealed bubbles around the top right corner, here the bottom left corner had the most unhealed bubbles, which I don't know how to explain. The sort of stringy, collapsed bubble thing reminds me of another series of iron glazes I'm working on which are also relatively low in alumina, and form ugly blisters when thick. I'd have thought lower alumina would mean lower viscosity, and therefore give the glaze a better chance of healing over. While I was writing this, I thought of a possible explanation, but I'll put it in a separate post.

The orange background appears with similar levels of Al2O3 for both tests. It would be interesting to do a grid varying silica and alumina around these points to see how far this region extends.

I love the variety of colours and combinations of colours that this tile produced. It's hard to pick a favourite. The area in the middle is the most striking (and is what got me started on this series of tests), but I'm also drawn to the yellow-purpley combination in bottom right corner. In fact, if you look closely, the purpley background has tiny areas of red, which might be encouraged by changing the cooling cycle.

large.IMG_0034.JPG.aa8b2c0ffd41ca70a88a30c89eb181b8.JPG

The top left glaze and the one to the right of it (#1 and 2) are really interesting, even though I'm not mad about the dirty yellowish - orangey background. I'm intrigued by the blue though, and will probably play with the amount of phosphorus to see how it effects this.

large.IMG_0033.JPG.99ae12749db83d82ebcd2e9a1b46cc46.JPG

But the glaze I keep coming back to is the third from the top in the left-hand column (#11). I really love the subtle blue halos.

large.IMG_0031.JPG.a4001def1da3c1de8b5e60b88e409963.JPG
 

Unfortunately the frit I used for these tests has been discontinued, and the replacement my supplier claims is the same is slightly different. Time to buy a big batch of frit from a supplier that provides the composition, if only I can find one.

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Here's what I think may be happening with the bottom left corner.

There's a 1914 BSc thesis by Sidney Sewell where he measures the viscosity of a family of glazes as a function of temperature. All glazes have 0.3 K2O, 0.7 CaO with Al2O3 ranging from 0.3 to 1 and SiO2 from 1.8 to 6 UMF. (Well, that was the plan, but the little slacker didn't finish testing all of them in time). He found that for fixed SiO2, decreasing Al2O3 increases the rate at which viscosity changes with temperature. So assuming this is also true for the family of glazes in my grid, as the kiln cools, the ones near the bottom increase in viscosity more rapidly than the ones near the top. So if there's a window of viscosities in which the glaze is too stiff to allow bubbles to grow, but fluid enough to heal over burst bubbles, the low alumina glazes may not be spending enough time in that window.

Of course, this doesn't explain why adding iron makes the bubbles heal over. Maybe in this situation it's acting like alumina?

I'd be interested in hearing any other explanations.

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@Pieter Mostert

After looking at the tiles for a while, I have to say my favorite tile is number 9. It looks like there is a mix of red and blueish spotting. It seems when the spots get bigger they have red in the center otherwise they look greenish blue. I know most of them have this same thing, but not on this darker background, which I find appealing. Really brilliant. As far as the chemistry and melt stuff goes, no idea. Hope you figure it out.

Edited by Joseph F
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So looking at the tile alumina increases up and iron to the right? The pattern certainly looks like it's designed by bubbles but the bottom left is confusing me, is there a second bubbling happening because the bottom left seems the most mature/melty. Lowering alumina may increase it's viscosity change but does that mean it gets less viscous at top temperature or more? I think maybe it's less viscous and something else could be going on there. Just my initial thoughts.

 

RE: Vertical/horizontal grids.

I still think the best way is to extrude L shaped sections for each row of the grid and stamp 5 squares in then brush the glaze onto the vertical section. Not a total match to dipping but close enough for me. One day I will get round to making this idea.

post-23281-0-37304000-1494879332.png

Edited by High Bridge Pottery
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Well, I got my slab roller setup this weekend and I went out and bought 2 dollars worth of kids play foam. I ended up building this:

IMG_20171126_090144.jpg.5eb20a4106cf10afa9badcf5b46be37c.jpg

 

Which is three square foams thick. The triangle on the bottom right being the highest point. I rolled out a slab this morning then put this over it and rolled it through:

DSCN6393.JPG.a60526c86975a6010a5acc490adbbf22.JPG

DSCN6394.JPG.e15c61b3a9c780dacaa4102df2bb36e8.JPG

I am pretty happy with it. This wasn't fully impressed as I adjusted the roller back up. This is a 1/4'' thick tile which I think is about right. I love how easy it is to make a tile with texture now. You can see the gentle slopes left by the foam and the ridges I have decided to use.

If this works well I can adapt even a better one maybe. I am hoping this removes the need for the vertical tiles, but if it doesn't I am making them today to see how all that is going to work as well. Just wanted to post this for an update to all my thoughts about the foam and roller stuff. Sorry if the pictures are blurry, I don't use my actual camera much anymore and my phone died.

Thanks to @Min for the article that gave me the information on what material to use! Worked well for version 1.

Edited by Joseph F
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8 minutes ago, Joseph F said:

Well, I got my slab roller setup this weekend and I went out and bought 2 dollars worth of kids play foam. I ended up building this:

IMG_20171126_090144.jpg.5eb20a4106cf10afa9badcf5b46be37c.jpg

 

Which is three square foams thick. The triangle on the bottom right being the highest point. I rolled out a slab this morning then put this over it and rolled it through:

DSCN6393.JPG.a60526c86975a6010a5acc490adbbf22.JPG

DSCN6394.JPG.e15c61b3a9c780dacaa4102df2bb36e8.JPG

I am pretty happy with it. This wasn't fully impressed as I adjusted the roller back up. This is a 1/4'' thick tile which I think is about right. I love how easy it is to make a tile with texture now. You can see the gentle slopes left by the foam and the ridges I have decided to use.

If this works well I can adapt even a better one maybe. I am hoping this removes the need for the vertical tiles, but if it doesn't I am making them today to see how all that is going to work as well. Just wanted to post this for an update to all my thoughts about the foam and roller stuff. Sorry if the pictures are blurry, I don't use my actual camera much anymore and my phone died.

Thanks to @Min for the article that gave me the information on what material to use! Worked well for version 1.

Looks interesting.  I can imagine three levels/depths of glaze, maybe cascading down from high to low if the glaze is fluxy, or sticking to three seperate levels if too stiff to run?

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26 minutes ago, curt said:

I can imagine three levels/depths of glaze, maybe cascading down from high to low if the glaze is fluxy, or sticking to three seperate levels if too stiff to run?

That is what I am hoping for. I am still going to run vertical tiles this first go around just to see how well that works as well. Once I figure out my trench blueprints and tile designs for vertical later this afternoon. 

4 hours ago, High Bridge Pottery said:

One day I will get round to making this idea.

I agree that this seems like a good solution, my issue is that your still going to have massive runs in the c corner to catch on those vertical tiles. I thought about this design for a good bit as well. It would definitely work for the majority of the grid, although I think brushing is slow. I would rather mix a slightly larger batch and dip, or find narrower cups maybe. I thought about just making my own cups out of clay so that I can label each cup with the grid corner amounts and also have a shape so that 40ml is tall enough to dip a tile, so a narrow tall cup. I am not sure how well it would work though. I like to look through the clear cups to make sure all the fluids are even across the cups. Although I have never adjusted one because of this.

I have data projects out the wazoo to work on this weekend so I am just rolling tiles today and I will be mixing the grids up next weekend. 

I have a small hand held extruder, but I haven't made any kind of L shaped tile. My plan is to roll out slabs, then cut uniform rectangles, mark them so they have a little texture, then fold them to a slant like:

 tile.png.3a2638cefaa036f2baa07878ef1176f1.png

This will be the shape of my main glaze tiles, it will also be the shape of my currie tiles in the trench, except it wont have the flat foot on the bottom since they will sit in the grid trench.

This way I slow down the flow of the glaze slightly since it will be on a slight incline.  

The green line indicates where I will put some marks for texture, and the red line will have some type of line so that I know that is where to dip the tile to. This way all the tiles will have uniform dipping distance so that when I fire them I can see the melt and flow of the glaze. I am probably going to make some kind of angle mold so that I can bend tiles to uniform bend every time. Probably will use foam for this as well. Quick and easy.

Trying to make all this as scientific as I can so that I can get the most information possible.

Edited by Joseph F
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On 25/11/2017 at 6:38 PM, Pieter Mostert said:

This time I also made a little hole in the glaze in each square (based on Curt's suggestion of scoring a 6 o'clock line), but I can't see any evidence of them.

I find the six o’clock mark useful when I am not sure which cells will melt - or how well - and want to have an ex-post indicator.   It also gives some indication of how well the glaze heals over.  Every cell on your grids appears very well melted so that is not surprising.  In fact I suspect that was kind of the point with the oil spotting (ie, to have very fluid melts) so mission accomplished.  

 

On 25/11/2017 at 6:38 PM, Pieter Mostert said:

I reduced the range of iron slightly, to run from 0.09 to 0.22 UMF from left to right

So Pieter just to be clear, is there iron in the far left column or not?  The range you quote above suggests there is?  And it does look like there is from the pics.  Just want to be sure.

 

On 25/11/2017 at 6:43 PM, Pieter Mostert said:

He found that for fixed SiO2, decreasing Al2O3 increases the rate at which viscosity changes with temperature. So assuming this is also true for the family of glazes in my grid, as the kiln cools, the ones near the bottom increase in viscosity more rapidly than the ones near the top. So if there's a window of viscosities in which the glaze is too stiff to allow bubbles to grow, but fluid enough to heal over burst bubbles, the low alumina glazes may not be spending enough time in that window.

Does this mean that bubbles are forming as the kiln cools?  I am having trouble understanding how, on the one hand the glaze is too stiff to allow bubbles, but on the other hand it is fluid enough to heal over?   Will think about this some more.

 

On 25/11/2017 at 6:43 PM, Pieter Mostert said:

Of course, this doesn't explain why adding iron makes the bubbles heal over. Maybe in this situation it's acting like alumina?

  I think you are on the right track on this.  Corner A (high alumina) is in the good glass region looking at the Glazy chart above.  Corner C (high iron low alumina) could be smooth because it is forming a different kind of “glass” due to the iron content which is more much more refractory in oxidation.  The way the iron “bubbles” to make the oilspots in every cell on the grid is different to the bubbles formed occasionally by off-gassing of other, non-iron clay or glaze materials.  

However, returning to corner C, I think the big bubbles there are due simply the massive amount of flux (and barely any alumina or silica) in that corner, probably low-melting flux (?), which is simply boiling and boiling away even long after the more “balanced” cells on the grid have stiffened up, and a lot of the way down as the kiln is cooling.  Maybe.  I don’t remember if you have posted the specific recipe or not, or what else is in the glaze.

Edited by curt
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Sorry for not responding sooner; I've had internet trouble at home over the weekend.

@Marcia Selsor Funny, I was looking at that paper on crystalline glazes yesterday. I know that the knowledge of crystalline glazes has advanced alot since then, but it's still a great source of data.

I think Ferro 3191 is reasonably similar to the frit I was using, but I'd have to import it. Will try some local options first.

@glazenerd I've noticed that in general, my tests on the red earthenware are more likely to pinhole than the ones on white stoneware, so that could well be the result of iron sulphide. But in this case I think the source of most of the gas and subsequent bubbling is from the decomposition of Fe2O3 to FeO. I can't explain why it would happen at 1130C instead of 1230C, which is the usual temperature given, but it still seems to me to be the most likely explanation.

@curt The oxides other than iron and alumina are the same as in this recipe. The left-hand column has 5 - 6 % red iron oxide, which translates to 0.09UMF.

The reason I think the bubbles may still be forming (or at least expanding and bursting) as the kiln cools, is that the later ones sometimes have different colour developing. For example, in the first close-up, the squashed bubble in the centre is more orange than the two adjacent to it, which must have formed later. This is assuming that the crystals giving the orange colour only form at lower temperatures.

I may be (probably am) completely wrong about the effect of changing viscosity on the bubbles. I guess I was thinking something along the lines of what Tony Hansen talks about here:

Quote

- Step 4: Use this step if you need to heal blisters. Free-fall 100F and hold. The reason: The lower temperature imposes the melt viscosity sufficiently to overcome melt surface tension and burst the bubbles but still affords enough fluidity to heal them.

I really need to go back and read the bubble thread again.

Edited by Pieter Mostert
Fixed typo
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@Joseph F When I increased the phosphorus in my original glaze (see link in my reply to curt) there was a more abrupt transition between the orange centre and the background colour. (The application on this tile was way too thick, but you get the picture)

large.l_8217.58cd839d1989e.jpg.10fd6d9b08cdd36aac32842dd8d7dc68.jpg

I think you'd have a similar result if you increased the phosphorus in #9.

@High Bridge Pottery Yes, alumina increases up and iron to the right. If the glazes in my grid behave similarly to the ones in the paper I mentioned, then for a given column, the ones lower down start softening at the same time as the ones higher up, but they're more fluid at the top temperature, so the rate at which the viscosity changes with temperature is greater. I think this sudden stiffening of the glaze as it cools is trapping the bubbles.

After looking at the paper again, I realised that the conclusion I quoted was for clear glazes only (so alumina can't be too high, as in the glazes I tested). Even here, the data isn't totally convincing.

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