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Making a mold


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#1 Strelnikov

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Posted 25 June 2013 - 10:55 AM

I am trying to make my first mold and have made a wood box to surround the mold. I have some Murphy's Oil Soap to use as a release agent on the inner part of the mold. I was wondering if I could line my wood box with a black plastic garbage bag so the USG No 1 Pottery Plaster doesn't leak out of the wood box. I have read that the plaster heats up when it's curing. Will it get too hot and melt the plastic?

#2 Benzine

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Posted 25 June 2013 - 10:59 AM

If you clamp the box tight enough, it shouldn't be an issue. Otherwise, you could use coils/ wads of clay to act as a seal, on the inside corners.
"Anything worth believing, is worth questioning"

#3 Strelnikov

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Posted 25 June 2013 - 11:10 AM

If you clamp the box tight enough, it shouldn't be an issue. Otherwise, you could use coils/ wads of clay to act as a seal, on the inside corners.


Thanks, that would work but would take more time than just putting a black plastic garbage bag inside the box. Just wondering if anyone has tried that successfully.

#4 Benzine

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Posted 25 June 2013 - 11:31 AM


If you clamp the box tight enough, it shouldn't be an issue. Otherwise, you could use coils/ wads of clay to act as a seal, on the inside corners.


Thanks, that would work but would take more time than just putting a black plastic garbage bag inside the box. Just wondering if anyone has tried that successfully.


Can't say I have. In regards to your question about the heat, there have been cases of people being burned by plaster, especially when doing plaster wrappings of body parts. Though, some of that, I believe, has to do with the plaster reacting with the moisture in the skin.
"Anything worth believing, is worth questioning"

#5 trina

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Posted 25 June 2013 - 11:34 AM


If you clamp the box tight enough, it shouldn't be an issue. Otherwise, you could use coils/ wads of clay to act as a seal, on the inside corners.


Thanks, that would work but would take more time than just putting a black plastic garbage bag inside the box. Just wondering if anyone has tried that successfully.



Yes you should have no problem using the bag. I have done it. It does make for a less professional looking mold though as it is hard to keep all the wrinkles out of the bag. T

#6 Strelnikov

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Posted 25 June 2013 - 11:47 AM

Yes you should have no problem using the bag. I have done it. It does make for a less professional looking mold though as it is hard to keep all the wrinkles out of the bag. T


Thanks Trina, at this point I don't care much what the outside of the mold looks like, just the inside. I'm most concerned about the plaster sticking to my wood box. I had to use glued pine for the bottom because it's wider than 11-1/4" so I couldn't use a piece of the 1 x 12 I used for the sides. The sides varnished up nice and smooth but for some reason the bottom pitted when I applied the varnish. Very strange but I won't use that stuff again. Although clay would work, a plastic garbage bag is sure faster and easier to solve the problem of sticking to the rough bottom.

#7 TJR

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Posted 25 June 2013 - 11:52 AM

I agree with Trina. You mold will look nicer if you put coils of clay along all the edges, both inside and outside. Your plastic bag may catch on your mold and chip. I use the spray on Murphy's oil soap. Works great. Don't worry about the plaster heating up. Not hot enough to melt anything.
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Throw your clay away after use. It will be no good for making anything.
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#8 trina

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Posted 25 June 2013 - 11:55 AM

I agree with Trina. You mold will look nicer if you put coils of clay along all the edges, both inside and outside. Your plastic bag may catch on your mold and chip. I use the spray on Murphy's oil soap. Works great. Don't worry about the plaster heating up. Not hot enough to melt anything.
TJR.
Throw your clay away after use. It will be no good for making anything.
T.



Is your box an actual box? or a frame? You are still good with the bag either way just curious. T

#9 OffCenter

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Posted 25 June 2013 - 12:05 PM

Are you using cottle boards? Having those around makes it easy to make molds and every time you use them they get better because of the Murphy's soap sinking in. A bead of clay to seal the cracks is better than a bag. Even if you don't seal it with clay and it leaks a little, properly mixed plaster will self seal after spreading out a bit. (Note the word "little" in previous sentence.) Avoiding air bubbles will probably be a bigger problem than any of your present concerns.

Jim
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#10 Marcia Selsor

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Posted 25 June 2013 - 12:28 PM

As Jim says, if you get into making molds cottle boards are a great asset. I seal mine with urethane.
I have several sizes. They are great for preparing for casting molds.

I agree with Trina that the plastic bag could make wrinkles and be a bear to get the plastic free. You could spray
the box with WD 40. Thta's faster than the bag. Benzines suggestion for coils at the seams is standard procedure for frames. But your box sounds all inclusive.

Marcia

#11 Strelnikov

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Posted 25 June 2013 - 04:22 PM


I agree with Trina. You mold will look nicer if you put coils of clay along all the edges, both inside and outside. Your plastic bag may catch on your mold and chip. I use the spray on Murphy's oil soap. Works great. Don't worry about the plaster heating up. Not hot enough to melt anything.
TJR.
Throw your clay away after use. It will be no good for making anything.
T.



Is your box an actual box? or a frame? You are still good with the bag either way just curious. T


It's a wood box made from 1" pine held together with wood screws.

Not sure about the bag now. Since this is my first mold my biggest fear is having everything stick together and having to destroy my box to free the mold. I would like to make several molds using this box.

#12 Strelnikov

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Posted 25 June 2013 - 04:25 PM

Are you using cottle boards? Having those around makes it easy to make molds and every time you use them they get better because of the Murphy's soap sinking in. A bead of clay to seal the cracks is better than a bag. Even if you don't seal it with clay and it leaks a little, properly mixed plaster will self seal after spreading out a bit. (Note the word "little" in previous sentence.) Avoiding air bubbles will probably be a bigger problem than any of your present concerns.

Jim


Never heard of cottle boards before, just have a wood box.

#13 Strelnikov

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Posted 25 June 2013 - 04:31 PM

As Jim says, if you get into making molds coddle boards are a great asset. I seal mine with urethane.
I have several sizes. They are great for preparing for casting molds.

I agree with Trina that the plastic bag could make wrinkles and be a bear to get the plastic free. You could spray
the box with WD 40. Thta's faster than the bag. Benzines suggestion for coils at the seams is standard procedure for frames. But your box sounds all inclusive.

Marcia


I sealed the inside of the box with 3 coats of Minwax Indoor/Outdoor Helmsman Spar Urethane. I'm concerned that the box is not straight, that the top is slightly smaller than the base, making the mold impossible to get out without destroying the box. Maybe lining the box with clay would be the best after all, so I could make sure the box has the correct taper to allow the plaster to come out of the box.

WD 40 is one release agent I've seen referenced, but more people seem to use Murphy's Oil Soap.

#14 OffCenter

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Posted 25 June 2013 - 04:35 PM


Are you using cottle boards? Having those around makes it easy to make molds and every time you use them they get better because of the Murphy's soap sinking in. A bead of clay to seal the cracks is better than a bag. Even if you don't seal it with clay and it leaks a little, properly mixed plaster will self seal after spreading out a bit. (Note the word "little" in previous sentence.) Avoiding air bubbles will probably be a bigger problem than any of your present concerns.

Jim


Never heard of cottle boards before, just have a wood box.


If you plan to make more than one mold, you should google cottle boards. Then google "do a barrel roll".

Jim
E pur si muove.

"But it does move," said Galileo under his breath.

#15 Strelnikov

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Posted 25 June 2013 - 05:57 PM

If you plan to make more than one mold, you should google cottle boards. Then google "do a barrel roll".

Jim


Will do thanks.

#16 trina

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Posted 26 June 2013 - 07:59 AM



Are you using cottle boards? Having those around makes it easy to make molds and every time you use them they get better because of the Murphy's soap sinking in. A bead of clay to seal the cracks is better than a bag. Even if you don't seal it with clay and it leaks a little, properly mixed plaster will self seal after spreading out a bit. (Note the word "little" in previous sentence.) Avoiding air bubbles will probably be a bigger problem than any of your present concerns.

Jim


Never heard of cottle boards before, just have a wood box.


If you plan to make more than one mold, you should google cottle boards. Then google "do a barrel roll".

Jim


thank you Jim I couldn't think of the word for cottle boards....brain spazm..T

#17 Strelnikov

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Posted 26 June 2013 - 10:28 AM


If you plan to make more than one mold, you should google cottle boards. Then google "do a barrel roll".

Jim


Will do thanks.


Found a couple of good videos on YouTube showing how to make and use cottle boards.

The barrel roll didn't work on my iPad but I saw a video showing it.

I thought you got sidetracked and were talking about airplanes and air shows.

#18 OffCenter

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Posted 26 June 2013 - 11:24 AM



If you plan to make more than one mold, you should google cottle boards. Then google "do a barrel roll".

Jim


Will do thanks.


Found a couple of good videos on YouTube showing how to make and use cottle boards.

The barrel roll didn't work on my iPad but I saw a video showing it.

I thought you got sidetracked and were talking about airplanes and air shows.


Yeah, I was just joking about the barrel rolls. Sorta cool that Google has a sense of humor. I don't know what you're doing but if you plan to make more molds cottle boards are well worth the effort to make them. It is so easy to adjust them to so many different sizes. I have two sets. One is for small molds and the other is for large molds. Good luck.

Jim
E pur si muove.

"But it does move," said Galileo under his breath.

#19 Strelnikov

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Posted 26 June 2013 - 04:48 PM

One thing to consider is that I made my box out of cheap pine that wasn't very straight. The video I watched on making cottle boards (from Ceramic Arts Daily) said to use oak or birch plywood. That stuff has to be pretty expensive comparatively if I even knew where to get it. Home Depot has oak boards but they are very expensive. I don't remember ever seeing oak or birch plywood there. I guess the good thing is that cottle boards are adjustable and once you bear the expense of making them, they can be used many times.

Another problem is that I don't have a suitable base to put the cottle boards on. In one video the instructor suggested using something like a formica counter base I believe. What are you folks using?

#20 OffCenter

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Posted 26 June 2013 - 10:36 PM

One thing to consider is that I made my box out of cheap pine that wasn't very straight. The video I watched on making cottle boards (from Ceramic Arts Daily) said to use oak or birch plywood. That stuff has to be pretty expensive comparatively if I even knew where to get it. Home Depot has oak boards but they are very expensive. I don't remember ever seeing oak or birch plywood there. I guess the good thing is that cottle boards are adjustable and once you bear the expense of making them, they can be used many times.

Another problem is that I don't have a suitable base to put the cottle boards on. In one video the instructor suggested using something like a formica counter base I believe. What are you folks using?


No need to use oak or birch, just use plain old pine or any other inexpensive wood. Mine are pine and are nice and straight and have been used for years and only get better with age. Just use any wood that is straight and smooth. You can set them on any work table. I spray a little WD40 on the table first then use the cottle boards. If I want an especially smooth bottom to the mold I set my cottle boards on my grand piano.

Jim
E pur si muove.

"But it does move," said Galileo under his breath.




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