William Brouillard on wed 6 nov 96
One of the students in my third year class, (1st. yr. as a clay major)
has asked me to post this question to Clayart.
They are doing clay body research and testing. In each test the clay body is
formed into a bar approx. 11" x 2.5" x .5". On that bar a line 10"
inches long is inscribed. The line is measured while the clay bar is
wet, bone dry, and after being fired to final temperature. To determine
total shrinkage, (wet to fired) shrinkage the fired length is subtracted
from the wet length. The difference is the total shrinkage , wet to fired.
Original length = 10
fired length = 8.5
total shrinkage = 1.5" or 15%
How would I write a formula for figuring the shrinkage of any length
of line.
Example
wet length of line in inches =W
fired length of line in inches = F
shrinkage in inches = S
W - F = S
For William Mudd
Cleveland Institute of Art
Replys can be sent to me or to the list. Thank you.
--
william brouillard
1011 literary road
cleveland,oh.44113
ch151@cleveland.freenet.edu
William Amsterlaw on fri 8 nov 96
William Brouillard asked for William Mudd:
>>
How would I write a formula for figuring the shrinkage of any length
of line.
<<
You need an expression that relates the change in length to the original wet
length. This is can be done by expressing the change in length as a fraction
of or percentage of the original wet length (the "percent shrinkage").
If W=wet length and F=fired length, then the change in length is S=W-F and:
%shrinkage = 100% * S / W
- Bill Amsterlaw (wamster@msn.com)
Plattsburgh, NY
Jean Lehman on sat 9 nov 96
Hi --
I figured there would be lots of answers on this, but apparently the list
is still down. At least I have no mail since Wednesday.
My statistician husband says the formula would be
W * (100-S)=F
Or, simply multiply any wet lenth by .85 to get the fired length for your
example of 15% shrinkage for your particular clay.
Jean Lehman, in Lancaster, PA
married to Dick Lehman (the statistician/glass blower, NOT the potter).
j_lehman@acad.FandM.EDU (that's an _underscore_ not a hyphen)
>Original length = 10
>
>fired length = 8.5
>
>total shrinkage = 1.5" or 15%
>
>
>
>How would I write a formula for figuring the shrinkage of any length
>
>of line.
>
>
>
>Example
>
>wet length of line in inches =W
>
>fired length of line in inches = F
>
>shrinkage in inches = S
>
>
>
>W - F = S
>
>
>
>For William Mudd
>
>Cleveland Institute of Art
>
>
>Replys can be sent to me or to the list. Thank you.
>
>--
>william brouillard
>1011 literary road
>cleveland,oh.44113
>ch151@cleveland.freenet.edu
Patsy Catsos on sat 9 nov 96
I took a little more empirical approach:
I cut a bunch of test tiles, and measured them before and after firing. I
graphed these numbers on an X-Y axis where x=length before firing, and
y=length after firing. Plotted the before/after points. Connected the
points with a straight line (it is a linear function). I use the chart to
convert before/after measurements.
Patsy Catsos
Craig Martell on sun 10 nov 96
Formulas for clay shrinkage and absorbtion can be found in Clay and Glazes
for the Potter by Daniel Rhodes pages 310-311. If you wanna know this, I
make my shrinkage bars 10cm long (100mm). Every mm equals 1% shrinkage,and
you don't need to do any math to figure this out. I test my clays
periodically and I always test 10 bars and average the results of all 10. I
also put cones next to the test bars to determine the heat work done. etc.
etc.
Kind Regards, Craig Martell-Oregon
John Post on sun 10 nov 96
Here is how I calculate the shrinkage of various clay bodies. I make a
small slab and mark a line on it that is 10 centimeters long. I fire the
slab and remeasure the line. If the 10 cm line is now 8.8 cm then that
means that the rate of shrinkage is 12%. (100mm - 88mm = 12mm; 12mm =12%).
If I wanted to make a slab that was 6 inches x 6 inches after firing and the
clay I was using had a shrinkage rate of 12%, I would use the following
formula...
Finished slab: 6" x 6"
Shrinkage: 12%
M = Original moist clay slab size
S = Percentage of shrinkage
F = Finished slab size
Use the formula (1-S)M = F
(1-12%)M = 6
(0.88)M = 6
M = 6 / 0.88
M = 6.8"
So in order to make a slab that is 6" x 6" when fired, it will have to be
6.8" x 6.8" in moist clay.
I hope this helps.
John
Dave and Pat Eitel on mon 11 nov 96
>----------------------------Original message----------------------------
>Here is how I calculate the shrinkage of various clay bodies. I make a
>small slab and mark a line on it that is 10 centimeters long. I fire the
>slab and remeasure the line. If the 10 cm line is now 8.8 cm then that
>means that the rate of shrinkage is 12%. (100mm - 88mm = 12mm; 12mm =12%).
>
>If I wanted to make a slab that was 6 inches x 6 inches after firing and the
>clay I was using had a shrinkage rate of 12%, I would use the following
>formula...
>
>Finished slab: 6" x 6"
>Shrinkage: 12%
>
>M = Original moist clay slab size
>S = Percentage of shrinkage
>F = Finished slab size
>
>Use the formula (1-S)M = F
>
>(1-12%)M = 6
>(0.88)M = 6
>M = 6 / 0.88
>M = 6.8"
>
>So in order to make a slab that is 6" x 6" when fired, it will have to be
>6.8" x 6.8" in moist clay.
>
>I hope this helps.
>
>John
This is one of the reasons I'm a potter and not a mathematician--I don't
get why one can't just add 12% to the finished size --which, in this
example, would make the slab 6.72 --which is obvioiusly the wrong answer (I
guess). I've been doing more or less that for more years than I can
calculate and I'm usually in the ballpark.
By the way I don't want to repoen that mathematics thread from a few weeks back!
Later...Dave
Dave Eitel
Cedar Creek Pottery
Cedarburg, WI
pots@cedarcreekpottery.com
http://www.cedarcreekpottery.com
John Post on wed 13 nov 96
Dave,
There are two ways to look at this math calculation problem. One is to
accept the formula, write it down and use it by plugging in the numbers that
are relevant to your clay work. The other is to look up some algebra
tutorial books containing information about ratios and proportions and gain
a working understanding of this concept by actually doing some calculations.
The reason your method is not correct is that you are multiplying the
finished fired slab by 12%. The 12% shrinkage refers to how much the
original moist slab will shrink. I realize that you are in the ballpark
using the way you calculate, but that is because you are working with small
numbers. Bigger numbers will reveal bigger discrepancies in the calculation
Another way that artists use this same type of ratio/proportion calculation
is when they are pricing work. Let's say I sell my work at a gallery that
takes a 40% commission on the sale price. How would I determine how much to
ask for a piece if I want to receive 100 dollars as my part of the sale?
Using your method, I would just multiply $100 by 40% and then add that
number to $100. That would mean the price of the piece in the gallery would
be $140.00. Unfortunately, when the gallery takes its 40% of the sale price
of $140 that equals $56, leaving me with only $84. Again, the correct way
to do this calculation is with algebra/ratios.
60/100 = the 60% part of the sale that I recieve
$100 = how much I want to make on the piece
X = how much the gallery will have to charge
60/100 = $100/X
60 * X = 100 * 100
60X = 10000
X = 10000 * 1/60
X = 166.66
Therefore, the gallery has to charge $166.67 for me to get my $100. I know
this looks complicated, but expressing ratios and percentages as fractions
using a keyboard isn't pretty. Believe me, if you get an algebra book this
stuff is pretty straightforward and simple.
John
>>----------------------------Original message----------------------------
>>This is one of the reasons I'm a potter and not a mathematician--I don't
>>get why one can't just add 12% to the finished size --which, in this
>>example, would make the slab 6.72 --which is obvioiusly the wrong answer (I
>>guess). I've been doing more or less that for more years than I can
>>calculate and I'm usually in the ballpark.
>>
>>By the way I don't want to repoen that mathematics thread from a few weeks
back!
>>
>>Later...Dave
>>
>>Dave Eitel
>>Cedar Creek Pottery
>>Cedarburg, WI
>>pots@cedarcreekpottery.com
>>http://www.cedarcreekpottery.com
>>
>>----------------------------Original message----------------------------
>>Here is how I calculate the shrinkage of various clay bodies. I make a
>>small slab and mark a line on it that is 10 centimeters long. I fire the
>>slab and remeasure the line. If the 10 cm line is now 8.8 cm then that
>>means that the rate of shrinkage is 12%. (100mm - 88mm = 12mm; 12mm =12%).
>>
>>If I wanted to make a slab that was 6 inches x 6 inches after firing and the
>>clay I was using had a shrinkage rate of 12%, I would use the following
>>formula...
>>
>>Finished slab: 6" x 6"
>>Shrinkage: 12%
>>
>>M = Original moist clay slab size
>>S = Percentage of shrinkage
>>F = Finished slab size
>>
>>Use the formula (1-S)M = F
>>
>>(1-12%)M = 6
>>(0.88)M = 6
>>M = 6 / 0.88
>>M = 6.8"
>>
>>So in order to make a slab that is 6" x 6" when fired, it will have to be
>>6.8" x 6.8" in moist clay.
>>
>>I hope this helps.
>>
>>John
>
>
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