John Rodgers on wed 29 dec 04
OOPS!!!
Pardon the fuzzy math here. 2000 cu. in. minus 250 cu in is 1750 cubic
inches. NOT 1500 cu in.
Regards,
John Rodgers
Chelsea, AL
John Rodgers wrote:
> Craig, this works, but not always.
>
> I work with really odd shaped models at times, models so oddly shaped
> there is no way to determine the amount of plaster required through
> measuring XYand Z dimensions and calculating volumes. To arrive at
> volumes when working with really odd shapes, I do it two ways,
> depending on the model and the material is is made from.
>
> Archimedes and his principle works well.
>
> I place a bucket in a large pan. Fill the bucket with water until it
> is level with the top edge of the bucket, then slowly submerge the
> model into the bucket of water. The model displaces an amount of water
> equal to it's own volume. This volume of water that runs over the
> sides of the bucket is caught by the pan. I then pour the water into a
> rectangular pan, and measure the length, width and depth of the water
> in the pan. From the LWD numbers I can calculate the volume of water
> in cubic inches and equate that to the volume of plaster and thus to
> weight of plaster that *will not* be included in the mold. In the case
> of a two sided mold, eg, a mold with a right and left half, I must
> calculate the amount of plaster for 1/2 a mold times 2.
>
> I measure the molding area - Length, width, depth, and then calculate
> the volume. Next I subtract the amount of volume of 1/2 of the water
> from the molding area volume. The remainder will be the volume of
> plaster mix required to make one side of the mold.
>
> Example: I mold a vase (carved from wax). I do the archimedes thing
> and I get a volume of water, say 500 cubic inches. I then measure my
> molding area for one side of the vase mold. When the measurements are
> calculated, it comes to 2000 cubic inches. Since the vase volume is
> 500 cu in. we need only 1/2 that amount for one side of the mold, so
> 500 devided by 2 gives 250. Subtract the 250 from the 2000 and the
> remainder is 1500. This is the volume amount of plaster required for
> make one half of the mold. That volume must be converted to pounds to
> mix with water. Keep in mind however, that this is not dry plaster,
> but plaster casting slurry, after the water is added. So the use of
> the plaster Volume Mix Calculator is advised.
>
> Sometimes when the object to be molded is not suited for dumping in
> water, I use dried peas to work out the volumes. it works.
>
> Regards,
>
> John Rodgers
> Chelsea, AL
>
>
>
> Donalson wrote:
>
>> While I haven't poured quite as much plaster as Craig Clark in the
>> past year I have done a significant amount. Weighing the plaster is
>> the way to go as far as I am concerned. I purchased an inexpensive
>> digital platform scale on Ebay for this purpose.
>> Determining the amount of plaster you will need: the mathematical
>> approach. This is really pretty simple and doesn't hurt at all.
>> Once you get the hang of it and reap the benefits of not having
>> mountains of excess plaster to dispose of you too will be a convert.
>> Once your mold boards are set up and clay is in place you will need
>> to measure the volume that is going to be filled in by the plaster
>> during the first pour. For our purposes we are going to be measuring
>> cubic inches. Say our mold boards form a rectangle that is 10" x 20"
>> and wants to be 2" thick: the calculations are 10 x 20= 200 x 2= 400
>> cubic inches. Okay, now that we know we are going to be mixing up
>> enough plaster to "occupy" 400 cubic inches we simply divide the
>> number 400 by 80. That just happens to work out to 5 in this
>> case...5 what... you may be wondering...well 5 in this example is how
>> many QUARTS of water we will need for this specific pour. Each quart
>> of water needs 2 pounds 14 ounces of plaster for a consistency of
>> 70. Continuing the example above...we have determined we need 5
>> quarts of water for our pour. When we multiply this 5 quarts by 2
>> pounds 14 ounces, we come up with 14 pounds and 6 ounces. So in this
>> example we would mix 5 quarts of water with 14 pounds 6 ounces of
>> plaster and this would be just enough plaster to fill the
>> void..nothing more nothing less.
>>
>> Some smart person calculated that a quart of water and 2 pounds 14
>> ounces of plaster mixed will occupy 80 cubic inches...works every time.
>>
>> As JB would say "hope this helps".
>>
>> Craig AZ
>>
>> ______________________________________________________________________________
>>
>> Send postings to clayart@lsv.ceramics.org
>>
>> You may look at the archives for the list or change your subscription
>> settings from http://www.ceramics.org/clayart/
>>
>> Moderator of the list is Mel Jacobson who may be reached at
>> melpots@pclink.com.
>>
>>
>>
>>
>
> ______________________________________________________________________________
>
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
>
>
Donalson on wed 29 dec 04
While I haven't poured quite as much plaster as Craig Clark in the past =
year I have done a significant amount. Weighing the plaster is the way =
to go as far as I am concerned. I purchased an inexpensive digital =
platform scale on Ebay for this purpose. =20
Determining the amount of plaster you will need: the mathematical =
approach. This is really pretty simple and doesn't hurt at all. Once =
you get the hang of it and reap the benefits of not having mountains of =
excess plaster to dispose of you too will be a convert. Once your mold =
boards are set up and clay is in place you will need to measure the =
volume that is going to be filled in by the plaster during the first =
pour. For our purposes we are going to be measuring cubic inches. Say =
our mold boards form a rectangle that is 10" x 20" and wants to be 2" =
thick: the calculations are 10 x 20=3D 200 x 2=3D 400 cubic inches. =
Okay, now that we know we are going to be mixing up enough plaster to =
"occupy" 400 cubic inches we simply divide the number 400 by 80. That =
just happens to work out to 5 in this case...5 what... you may be =
wondering...well 5 in this example is how many QUARTS of water we will =
need for this specific pour. Each quart of water needs 2 pounds 14 =
ounces of plaster for a consistency of 70. Continuing the example =
above...we have determined we need 5 quarts of water for our pour. When =
we multiply this 5 quarts by 2 pounds 14 ounces, we come up with 14 =
pounds and 6 ounces. So in this example we would mix 5 quarts of water =
with 14 pounds 6 ounces of plaster and this would be just enough plaster =
to fill the void..nothing more nothing less.
Some smart person calculated that a quart of water and 2 pounds 14 =
ounces of plaster mixed will occupy 80 cubic inches...works every time.
As JB would say "hope this helps".
Craig AZ
John Rodgers on wed 29 dec 04
Craig, this works, but not always.
I work with really odd shaped models at times, models so oddly shaped=20
there is no way to determine the amount of plaster required through=20
measuring XYand Z dimensions and calculating volumes. To arrive at=20
volumes when working with really odd shapes, I do it two ways, depending =
on the model and the material is is made from.
Archimedes and his principle works well.
I place a bucket in a large pan. Fill the bucket with water until it is=20
level with the top edge of the bucket, then slowly submerge the model=20
into the bucket of water. The model displaces an amount of water equal=20
to it's own volume. This volume of water that runs over the sides of the =
bucket is caught by the pan. I then pour the water into a rectangular=20
pan, and measure the length, width and depth of the water in the pan.=20
From the LWD numbers I can calculate the volume of water in cubic=20
inches and equate that to the volume of plaster and thus to weight of=20
plaster that *will not* be included in the mold. In the case of a two=20
sided mold, eg, a mold with a right and left half, I must calculate the=20
amount of plaster for 1/2 a mold times 2.
I measure the molding area - Length, width, depth, and then calculate=20
the volume. Next I subtract the amount of volume of 1/2 of the water=20
from the molding area volume. The remainder will be the volume of=20
plaster mix required to make one side of the mold.
Example: I mold a vase (carved from wax). I do the archimedes thing and=20
I get a volume of water, say 500 cubic inches. I then measure my molding =
area for one side of the vase mold. When the measurements are=20
calculated, it comes to 2000 cubic inches. Since the vase volume is 500=20
cu in. we need only 1/2 that amount for one side of the mold, so 500=20
devided by 2 gives 250. Subtract the 250 from the 2000 and the remainder =
is 1500. This is the volume amount of plaster required for make one half =
of the mold. That volume must be converted to pounds to mix with water.=20
Keep in mind however, that this is not dry plaster, but plaster casting=20
slurry, after the water is added. So the use of the plaster Volume Mix=20
Calculator is advised.
Sometimes when the object to be molded is not suited for dumping in=20
water, I use dried peas to work out the volumes. it works.
Regards,
John Rodgers
Chelsea, AL
Donalson wrote:
>While I haven't poured quite as much plaster as Craig Clark in the past =
year I have done a significant amount. Weighing the plaster is the way t=
o go as far as I am concerned. I purchased an inexpensive digital platfor=
m scale on Ebay for this purpose. =20
>
>Determining the amount of plaster you will need: the mathematical appro=
ach. This is really pretty simple and doesn't hurt at all. Once you get=
the hang of it and reap the benefits of not having mountains of excess p=
laster to dispose of you too will be a convert. Once your mold boards ar=
e set up and clay is in place you will need to measure the volume that is=
going to be filled in by the plaster during the first pour. For our pur=
poses we are going to be measuring cubic inches. Say our mold boards for=
m a rectangle that is 10" x 20" and wants to be 2" thick: the calculation=
s are 10 x 20=3D 200 x 2=3D 400 cubic inches. Okay, now that we know we a=
re going to be mixing up enough plaster to "occupy" 400 cubic inches we s=
imply divide the number 400 by 80. That just happens to work out to 5 in=
this case...5 what... you may be wondering...well 5 in this example is h=
ow many QUARTS of water we will need for this specific pour. Each quart o=
f water needs 2 pounds 14 ounces of plaster for a consistency of 70. Con=
tinuing the example above...we have determined we need 5 quarts of water =
for our pour. When we multiply this 5 quarts by 2 pounds 14 ounces, we c=
ome up with 14 pounds and 6 ounces. So in this example we would mix 5 qu=
arts of water with 14 pounds 6 ounces of plaster and this would be just e=
nough plaster to fill the void..nothing more nothing less.
>
>Some smart person calculated that a quart of water and 2 pounds 14 ounce=
s of plaster mixed will occupy 80 cubic inches...works every time.
>
>As JB would say "hope this helps".
>
>Craig AZ
>
>________________________________________________________________________=
______
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pcli=
nk.com.
>
>
> =20
>
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