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Chris Campbell

Slab Roller Experiment

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While presenting at the Potters Council Handbuilding conference at the Spruill Center in Atlanta,

my students and I began discussing the effects we believed a slab roller had on the clay it was compressing and extending.

 

We were wondering just what was happening to both sides of the clay and what effect being compressed and stretched mainly

in one direction had on the clay. I kept wondering about this after I got home and decided to try a couple of simple experiments.

 

In order to see how the clay was moving through the rollers I decided to place some colored clay on both sides of a plain slice of clay.

Then. I ran it through my Bailey slab roller.

 

My website with more Images

 

After compressing and stretching the clay, I found each surface was different.

The pattern on the top of the clay had stretched 7" while the pattern on the bottom of the clay

had only stretched 4 1/2". Not much size difference side to side.

 

I decided to flip the piece and roll it again without changing anything else.

 

Now the design is 8" on the original top and 6.25 on the original bottom.

Obviously, the piece was compressed more on its length than its width.

 

So, I wondered what this meant to the shrinkage of a piece of work.

I cut it into a 6" by 6" tile and let it rest in the open air.

After 30 minutes, it had shrunk 1/16th inch on all sides.

After 3 hours it had shrunk 1/8th inch on all sides.

Perfectly even shrinkage with absolutely NO warping.

 

SO ... why did something that appeared to compress and stretch so unevenly shrink so evenly?

Is the conventional wisdom of rolling again across the grain really a necessary step if this tile stayed perfectly flat without any help?

 

My husband, the engineer, speculates slab rollers have more to do with compression than stretching so this is why it shrinks evenly.

 

Ideas or suggestions for another experiment are welcomed!

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Thanks for analysis Chris! It's cool to see how the clay is really moving. It's also refreshing to learn facts about clay (there's so much dogma floating around, I wish the culture of pottery would adopt an anti-dogmatic attitude).

 

I have something to add about stretching a slab across the grain. It has nothing to do with keeping a tile flat, but I do think it affects the shrinking behavior of clay during a cone 6 glaze firing.

 

I make a lot of square plates from slabs that are formed on slump molds. At first, I would open a new bag of clay and cut the amount I needed from the end of the block. Then I would stretch out the clay with the heel of my hand to about 9 inches wide, which is the width of the slab I need. Then I put it through my slab roller in the direction perpendicular to that 9 inch dimension.

 

One day, thinking I was being very clever, I realized I could remove the clay entirely from its plastic bag, lay it on its side, and cut off an amount of clay that was already 9 inches across. Then I could skip the step of stretching it out to 9 inches, and put it straight through my slab roller instead.

 

The plates appeared square throughout the whole process until I put them into a cone 6 glaze firing, when some of them would emerge as rectangles, because they shrank more in one dimension than the other. It took me a while to put it together with my clay cutting shortcut, but ever since I did I have been making reliably square plates again.

 

So stretching the clay in both dimensions does affect shrinkage at cone 6. But what's interesting is that a rudimentary stretching from 6 inches to 9 inches, done with the heel of the hand, provides the same amount of shrinkage regulation as a drastic and precise stretching from 6 inches to about 20 inches done with a slab roller.

 

-Mea

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love your site Chris, wish you had a rss feed to add to my reader though. Is in not a "blog" so much? Is that the reason no feed?

 

 

Probably has a lot to do with my not really understanding what is is and how best to use it.

I can add it though if I can understand how to use it.

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I am beginning to think that a slab roller is really a flat extruder rather than a stretcher.

It does give you a quick consistent flat slab with a desire to stay flat.

 

I am trying to figure out how best to create stripes for my next experiment so I can see what happens in the middle.

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Probably has a lot to do with my not really understanding what is is and how best to use it.

I can add it though if I can understand how to use it.

 

 

An RSS feed is a machine-readable page of article information (headlines, headlines and summaries, or headlines and full article) that a user can subscribe to with a "feed reader" (http://www.google.com/reader/ , for instance). I subscribe to over a hundred blogs, and I can see what's new on all of them by simply opening my feed reader.

 

Here in the CAD forum, if you look in the upper-right under the Search box, there's a link called "View New Content" and next to it is a little orange icon... that orange icon is the "feed" symbol. That gets you an RSS feed of threads with new content.

 

Your software (Sandvox Pro) seems to be rather "summary" oriented, with the assumption that readers will click through to visit your site. Most users of feed readers hate this... reading a hundred blogs, I want to read it like email... "read, next, skim, next," not have to visit each site to read the articles. It may be that you can specify a very large number for the "summary length" and get the full post. Don't know if that will include photographs, though. But even if it can't do that, headlines are better than nothing, because it tells me when you have new content, so I don't have to visit your page constantly just to find out if there's anything new.

 

Documentation on setting up an RSS feed in Sandvox:

 

http://docs.karelia.com/z/RSS_Feed.html

 

I hope that helps.

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RSS Feed might be working ....

my poor artistic brain is warped!! <G>

Let me know if it is actually working please.

 

 

That's close, Chris. It's lumping all the content under one article instead of separate articles. If your main page is really one big page of multiple articles (and not including sub-pages of one article each), your site might not be suited to a traditional RSS feed without a bit of a structural redesign. I'm not sure how it'll reflect changes in the feed... it may create a new article, or it might update the existing article. Some readers will notice changed articles and some will ignore them (and good readers give you the option to choose the behavior).

 

Thanks for giving it a try. I've added your feed to my reading list.

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Hi Chris, sorry to cause such a fuss and side track your original post. Once one starts using rss feeds (from reading point of view) they are invaluable for keeping up with "lots" of sites easily and daily.

 

Sort of like reading a daily newspaper that only has things one wants to read (terrible analogy) and like Carl says it makes it easy to keep up with 100's of site and see who has new posts just in the time it takes for morning coffee. Although it takes a bit more to go back and read the new posts

 

An easy way to see if a site has a feed is to look for the little orange feed icon on the right side in the browsers address bar when visiting a site.

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Thanks for analysis Chris! It's cool to see how the clay is really moving. It's also refreshing to learn facts about clay (there's so much dogma floating around, I wish the culture of pottery would adopt an anti-dogmatic attitude).

 

I have something to add about stretching a slab across the grain. It has nothing to do with keeping a tile flat, but I do think it affects the shrinking behavior of clay during a cone 6 glaze firing.

 

I make a lot of square plates from slabs that are formed on slump molds. At first, I would open a new bag of clay and cut the amount I needed from the end of the block. Then I would stretch out the clay with the heel of my hand to about 9 inches wide, which is the width of the slab I need. Then I put it through my slab roller in the direction perpendicular to that 9 inch dimension.

 

One day, thinking I was being very clever, I realized I could remove the clay entirely from its plastic bag, lay it on its side, and cut off an amount of clay that was already 9 inches across. Then I could skip the step of stretching it out to 9 inches, and put it straight through my slab roller instead.

 

The plates appeared square throughout the whole process until I put them into a cone 6 glaze firing, when some of them would emerge as rectangles, because they shrank more in one dimension than the other. It took me a while to put it together with my clay cutting shortcut, but ever since I did I have been making reliably square plates again.

 

So stretching the clay in both dimensions does affect shrinkage at cone 6. But what's interesting is that a rudimentary stretching from 6 inches to 9 inches, done with the heel of the hand, provides the same amount of shrinkage regulation as a drastic and precise stretching from 6 inches to about 20 inches done with a slab roller.

 

-Mea

 

Chris and Mea,

Both experiments are interesting. I , too have a bailey slab roller. I flip and turn my slabs for each pass through the rollers. I like slab rollers with dual rollers. I worked on a Brent for several years where the adjustable boards were on the bottom. I got uneven thickness for my slabs of 25" or so. The surface discrepancy is really interesting to think about. Thanks for the input.

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Chris: Nice experimnet - and very relevant to slab work. To find the answer to your problem maybe you need to go outside the window and make the clay warp. I would go back through the steps starting with the thickest your slab roller will handle then keep rolling it the same direction, the same side, not rotating, until it gets thin. Then dry it. I'd try it with porcelain too which has more plastic memory and see if you can make it warp.

 

- h -

 

 

While presenting at the Potters Council Handbuilding conference at the Spruill Center in Atlanta,

my students and I began discussing the effects we believed a slab roller had on the clay it was compressing and extending.

 

We were wondering just what was happening to both sides of the clay and what effect being compressed and stretched mainly

in one direction had on the clay. I kept wondering about this after I got home and decided to try a couple of simple experiments.

 

In order to see how the clay was moving through the rollers I decided to place some colored clay on both sides of a plain slice of clay.

Then. I ran it through my Bailey slab roller.

 

My website with more Images

 

After compressing and stretching the clay, I found each surface was different.

The pattern on the top of the clay had stretched 7" while the pattern on the bottom of the clay

had only stretched 4 1/2". Not much size difference side to side.

 

I decided to flip the piece and roll it again without changing anything else.

 

Now the design is 8" on the original top and 6.25 on the original bottom.

Obviously, the piece was compressed more on its length than its width.

 

So, I wondered what this meant to the shrinkage of a piece of work.

I cut it into a 6" by 6" tile and let it rest in the open air.

After 30 minutes, it had shrunk 1/16th inch on all sides.

After 3 hours it had shrunk 1/8th inch on all sides.

Perfectly even shrinkage with absolutely NO warping.

 

SO ... why did something that appeared to compress and stretch so unevenly shrink so evenly?

Is the conventional wisdom of rolling again across the grain really a necessary step if this tile stayed perfectly flat without any help?

 

My husband, the engineer, speculates slab rollers have more to do with compression than stretching so this is why it shrinks evenly.

 

Ideas or suggestions for another experiment are welcomed!

 

 

 

 

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