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  #1  
Old 08-24-2013, 03:50 PM
Art-Deco Art-Deco is offline
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Clay shrinkage

I'm going to have to figure out what the shrinkage of the red clay I use actually is, and allow for TWO shrinkages to wind up with the size I want to do pressed clay versions of my work.

I found my cherub plaque really loses a lot of it's visual impact due to the shrinkage. The original was 10" wide and the fired terracotta is only 8-7/8", that's quite a reduction.
So the way it has to work now is making a clay model large enough to compensate for it's own shrinkage plus the shrinkage of pressed clay copies of it, I'm not great at math enough to easily calculate what this will be per foot, here's some measurements I do have:

Original: 10"
Shrinkage: 8-7/8"

Original: 9-1/2"
Shrinkage: 8-1/2"

Original: 4-5/8"
Shrinkage: 4-1/4"

Best I can come up with is the clay shrinks one inch on a 9-1/2" original, so if I take 9-1/2" and divide it into four it is 2-1/8" which on a model that size would shrink 1/4"
Adding that to the 9-1/2 would be 11-5/8" and 1-1/4" shrinkage, I guess for my purposes then I can assume about 1-3/8" per foot for shrinkage.
I would have to allow for two times that for a clay model plus the resulting pressed clay casts.

I have a design I want to do next that measures 15" high, 10-5/8" wide at the top and 6-1/2" at the bottom, but none of those measurements are 12" or multiples of it, so I really kind of need to know what the mathematical formula is to figure out that 1-3/8" shrink over 12" what it is on 15" or any other non 12"/24"/36"/48" measurement.

The 15" is not too bad, as that's 12" plus one quarter of a foot (3")
so I can figure 1-3/8" + 1/4 of that but that still doesn't teach me the FORMULA to do it easier.

I come up with 2-1/8" shrinkage on 15" but double that means I'd have to make the original model 15" tall plus 4-1/4" to compensate for the double shrinkage to allow the fired pressed casts to end up just about the same 15" tall the original design is!
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  #2  
Old 08-24-2013, 05:24 PM
raspero raspero is offline
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Re: Clay shrinkage

Well. If you multiply your desired size of 8.5 inches by a factor of 1.118, what you get is 9.5 as the necessary wet size.

So, assuming that the shrinkage is fairly uniform over a span of sizes, if you want a piece, let's say a plaque 21.5 inches X 15 inches finished, you would make it 21.5 x 1.118 = 24.04 inches. X 15 x 1.118 = 16.77 inches. So 24 X 16 3/4 inches should shrink to the desired size.

All theoretically of course. The materials we use do play tricks on us.

Richard
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  #3  
Old 08-24-2013, 08:44 PM
Art-Deco Art-Deco is offline
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Re: Clay shrinkage

Thanks Richard, I think I see that, I'll have copy this and try it out, that's just what I needed.
I can live with variations caused by variances in the clay, if it's 1/4" either way that's fine, just wouldn't like to want to have 24" finished and start out with what I thought was the right oversize and wind up with 22", if I wound up with 23-3/4 or 24-1/4 that's totally fine.

Let's see, I took my 15" x 1.118 and it comes up to 16.77" which I assume represents ONE shrinkage, but remember- there will be TWO separate instances of shrinking going on because my original model when it dries will shrink (unless I make a mold of it while still moist) and then when it's dry I would be making a mold of that and pressing clay into that mold which also will shrink, so if I'm right my 15" x 1.118 = 16.77 x 1.118 for the second shrinking = 18.74". I set up this model to 19-1/4" so if that formula is right my form is maybe 1/2" taller than it needs to be, I can live with that and I can also shave a little off top and bottom of the damp model if I want to, I can also tweek that formula a little if it did wind up being that 1/2" off, at least it gives me a formula and starting point, and actually knowing the shrinkage per instance is good because there have been times Ive molded off a moist clay model that had no shrinkage to it yet.

Last edited by Art-Deco : 08-24-2013 at 09:04 PM.
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  #4  
Old 08-24-2013, 09:35 PM
Robson Valley Robson Valley is offline
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Re: Clay shrinkage

The three examples that you present show a shrinkage of approximately 10%.
Wet - 10% of wet dimension = dry/fired. I suggest that you might just scale up your designs by that amount, say 10 or maybe even 12%. Photocopiers have a magical ability to do that these days.

If and when I come up with a wood carving design that I like and carve, I'll stop in the city and make half a dozen different sizes in the Copy Center.
For my next trick, I want to scale up a turtle shell from 8" wide to 24". The little one is a home run. Lots of paper, might cost me a whopping $2 or $3.
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  #5  
Old 08-25-2013, 01:26 AM
Art-Deco Art-Deco is offline
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Re: Clay shrinkage

Quote:
Originally Posted by Robson Valley View Post
The three examples that you present show a shrinkage of approximately 10%.
Wet - 10% of wet dimension = dry/fired. I suggest that you might just scale up your designs by that amount, say 10 or maybe even 12%. Photocopiers have a magical ability to do that these days.

If and when I come up with a wood carving design that I like and carve, I'll stop in the city and make half a dozen different sizes in the Copy Center.
For my next trick, I want to scale up a turtle shell from 8" wide to 24". The little one is a home run. Lots of paper, might cost me a whopping $2 or $3.
Thanks for your input, that works too, though my stated measurements are only for ONE shrinking- the original clay model, there are TWO separate shrinkings involved, so in actuality it would seem to be closer to 20%

I would want say: 15" to be the final height after TWO clay shrinkings, one being the original model and the other being the hand-pressed clay cast.

I came up earlier with 19-1/4" which if correct and which even if I go by 20%, 20% of 15" is 3" which added up comes to only 18", so it looks like it would be closer to 28% or roughly 14% for each step, x 2 steps.

I know back in the 1890s era all of the architectural terracotta companies had finished fired sizes perfectly calculated for the clay they used and they made wood scale sticks for each clay. They were able to calculate it accurately enough that even very large works made up of multiple parts all fit within around 1/4" or less because they couldn't cut the fired terracotta blocks down, and if they were too large the masons would have to cut bricks around them to fit which cost additional labor, and if too small they would have to fill in the gaps with mortar which would look bad.

I don't use a photocopier since the only one available around here is at work to be used for work not personal projects, but I do have Vectorworks CAD software to print out the sizes I want. While the CAD has object scaling, it's somewhat funky the way it does it and it doesn't have the ability to scale height and width proportionally together except for entering a number to do it. I can "stretch" the height of the object and the width manually, but seperately, and doing it that way changes the proportions.
So for my purposes and methods, having a figure or formula to use and plug in is the easiest, be it 28% or better yet a measured number I can enter into the object size box as I would get by multiplying the measurements by 1.118 or whatever it actually turns out to be.

I'm aware too that different clays will be different, as well as firing to different cones, but I'm pretty much planning to use this one red clay exclusively, and always firing it to 2060 deg F- just shy of cone 1 to get the color I want, so the clay and the firing temperature will remain the same.
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  #6  
Old 08-25-2013, 09:55 AM
Robson Valley Robson Valley is offline
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Re: Clay shrinkage

Thanks for the terracotta information. Useful for a wood-head. There are production nuances which I would never consider until it was too late!
As the clay is amporphous, I guessed that the shrinkage would be the same in XYZ.

I could scan my best turtle drawings and attempt to change the size in the computer file. In the meantime, I'll just sit on them until I next get into the city.
Plan B: I bought the simple wooden pantograph from Lee Valley. Conserved the hardware and rebuilt it with 3/4" aluminum strips. Comfortably heavy now. I may have to burn the house down to find it.
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  #7  
Old 08-25-2013, 10:26 AM
Art-Deco Art-Deco is offline
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Re: Clay shrinkage

Quote:
Originally Posted by Robson Valley View Post
As the clay is amporphous, I guessed that the shrinkage would be the same in XYZ.

I could scan my best turtle drawings and attempt to change the size in the computer file. In the meantime, I'll just sit on them until I next get into the city.
Plan B: I bought the simple wooden pantograph from Lee Valley. Conserved the hardware and rebuilt it with 3/4" aluminum strips. Comfortably heavy now. I may have to burn the house down to find it.

Clay is an odd substance, it shrinks as we all know, but it can vary some too and it's not always consistant because it can be compressed, it can also warp, though the clay I use has an almost zero tendency to either warp or crack even though I've used it for a sculpture that had walls in some areas 4" thick. In theory it should shrink the same ratio across x, y, and z.

Here's a couple of screen shots of the CAD window, the first shows the scale objects window, easy to see you have to plug a number in such as that 1.118, used to scale objects proportionally but you do have to know what number to enter, or at least do it via trial and error:




This other window does it differently, you can set the layer to a certain scale for printing that size and then enter the actual measurements of objects selected, but you need to know both measurements because changing one doesn't automatically change the other- they are independent. This is the reason I needed a formula, the software doesn't figure this for me it needs input to do it.



Lee Valley has some good tools.
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  #8  
Old 08-25-2013, 11:53 AM
Robson Valley Robson Valley is offline
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Re: Clay shrinkage

Of course! Varying the water content would change the shrinkage. Glad it's your headache and not mine. Will this be a situation where you have to do some experiments to see reliable shrinkage data?

I found the pantograph, info and bits& pieces. Thank you for the engaging conversation. Has set a wood carving into motion that I've been mulling over in my head for months.
I have a piece of western red cedar log shell, is now about 22" x 24" x 5". The rotten core has been cleaned out with an adze and the 3/4" sap wood has been skinned off. One pencil-sized knot in a corner which I think I can avoid. Quite a chore to learn if the whole piece is worth keeping at all. Little bit more end trimming to do with a 36" bow saw.

Looks like a turtle body shell to me. I'll do the legs in some 2" x 8" birch plank, some of the nicest carving wood that I've ever used. Might do the head in either red or yellow cedar. Too many fine cracks to attempt a 1-piece work, too much wood wasted and the head and legs don't look like shell, anyway.
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  #9  
Old 08-25-2013, 02:22 PM
Art-Deco Art-Deco is offline
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Re: Clay shrinkage

Quote:
Originally Posted by Robson Valley View Post
Of course! Varying the water content would change the shrinkage. Glad it's your headache and not mine. Will this be a situation where you have to do some experiments to see reliable shrinkage data?
My first try of this double shrinkage compensation will be seat-of-the-pants on a working model, however it turns out that how it will be. This isn't critical but I'd like to get in the right ballpark and get this worked out so that in the future for pieces I do I can anticipate their final size, and also for clients who do need a specific size to fit a space such as in a brick wall and not have to cut bricks funny to get it to fit. 1/4 to 1/2" accuracy is probably close enough for my purposes, I just don't want to anticipate 24" and wind up with 22" in the end!



Quote:
I found the pantograph, info and bits& pieces. Thank you for the engaging conversation. Has set a wood carving into motion that I've been mulling over in my head for months.
I have a piece of western red cedar log shell, is now about 22" x 24" x 5". The rotten core has been cleaned out with an adze and the 3/4" sap wood has been skinned off. One pencil-sized knot in a corner which I think I can avoid. Quite a chore to learn if the whole piece is worth keeping at all.
Oh nice! this is for reduction/enlargement purposes? theres a lot of parts to those, I've seen pictures of old ones being used.
I've worked a lot with Western red cedar, it's very soft wood and seems to not want to hold glue real well in that you have to keep it clamped overnight unlike poplar and oak which are glued SOLID in 15 minutes to where you CANT pull it apart.
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  #10  
Old 08-25-2013, 04:04 PM
Robson Valley Robson Valley is offline
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Re: Clay shrinkage

I really like those brick wall accents. Can you carve that like other clays?
I was in Britain (N Yorks) a couple of summers ago, amazing how many of those things I found. Fine to look ahead but I had to learn to look back at what I had just passed.

The back side/wall of the Grosvenor Hotel faces out over the Victoria rail station in London. You can't see it unless you have reason to walk the causeway linking the old hotel with the newer addition (over the station.) Dozens of stone carvings, massive things (sandstone?) up around the perimeter, below the roof line.

Certainly necessary to make the finished unit size useful. Here in Canada, I've been told that brick layers just go nuts if they get metric brick to fix an old, Imperial-sized wall, say around a window.

Western Red Cedar is not without a lot of carving problems with long run-out splits if I don't make enough stop cuts. After 10 years, I think that I have finally learned the wood. I have done a couple of glue-ups for carvings but I work in fear. A big piece that cracks along a fault line in the middle of the work usually is not worth the fiddle to fix. Start over. I have to be fastidious about sweeping chips off the bench = work on one face while the chips are getting pounded into the soft wood of the other side.

Most of the time, I can see things in the wood = draw with a grease pencil and get busy. Other times, I get a drawing that I like and I need to fit it to the wood. Rebuilding the pantograph with aluminum was a good thing to do. It's heavy enough now to be quite stable, that was a major issue. I modified the arm end to hold an ordinary mechanical drafting pencil with 2B lead.

I got outside with the adze before it rained today, the log shell is now 20.5" outside curve, 13" inside curve and 22" tall. Maybe 4" thick, the carpenter ants really did a number on it. But for free junk, that's OK.
The pleasant surprise is that my best green sea turtle shell drawings will need a 2X enlargement to fit. I have a Banquet Roll, cheap paper table covering, 36" wide by 100' long, so size is no issue.

Really want to get outside and try some stone carving, dang rain showers.
I'll drop this turtle thing in a minute if the forecast improves. But it's the mountains = make their own local weather.
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  #11  
Old 08-25-2013, 05:41 PM
Art-Deco Art-Deco is offline
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Re: Clay shrinkage

Oh yes, that's stone, not sure what type of stone though, possibly limestone. Usually that stuff is carved in situ from scaffolding, I like the lion's expression. The building dates to 1860 and all kinds of interesting portrait roundels under the roof line. The lions are on the ground floor, looks like a white limestone or maybe even marble, I don't know what stone was commonly available in London:



Quote:
Certainly necessary to make the finished unit size useful. Here in Canada, I've been told that brick layers just go nuts if they get metric brick to fix an old, Imperial-sized wall, say around a window.
That is one issue another is getting the color right, there can be 100 different shades of red bricks and if replacements don't match they stand out. There's a mason here I've watched restore two brick buildings, he's a real master at it and does museum quality work.

What areyou going to make out of the cedar?
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  #12  
Old 08-25-2013, 11:02 PM
Robson Valley Robson Valley is offline
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Re: Clay shrinkage

Thanks for the picture, that's the sort of thing that I recall.

I would like to be able to post pictures of some carvings. All I have left is pictures, can't even remember what became of some carvings, they're not in the house.

Several members here have given me detailed instructions for making some sort of a photo account somewhere else. Then I need to learn how to post links in here? Is that right? I'm never successful at doing that.

I have just tried 10 times to open a Flickr account by opening a Yahoo account and it doesn't work. Their jerk arse software demands that I have a mobile number (wrong) and the same s/w does not believe that I know when I was born. Ten times is all the respect that I can possibly muster for those guys.
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  #13  
Old 08-25-2013, 11:49 PM
Art-Deco Art-Deco is offline
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Re: Clay shrinkage

Quote:
Originally Posted by Robson Valley View Post
Thanks for the picture, that's the sort of thing that I recall.

I would like to be able to post pictures of some carvings.

I have just tried 10 times to open a Flickr account by opening a Yahoo account and it doesn't work. Their jerk arse software demands that I have a mobile number (wrong) and the same s/w does not believe that I know when I was born. Ten times is all the respect that I can possibly muster for those guys.
You don't even need an account on http://www.imgur.com
It's free to uplaod pictures to and link to them, just be sure to copy the link it provides that has the [IMG] tags, and paste it in the text here, it offers other links like <img src=" etc don't use those, use the links that looks like this:

Code:
 [IMG]http://imgur.com/tyty1276VGU[/IMG ]
I totally hate flickr now, they changed the interface and it's absolutely horrible, it loads hundreds of these HUGE images all at once and drags your browser into lag hell while it chokes on trying to load all that garbage. You can open an imgur acct free, it's much easier, you can drag and drop images into it and arrange albums on your acct.
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