Need mold-making material advice

[Helenium]

I would like to make molds from natural forms such as tree trunks and rocks. Can anyone suggest a product that will not harm the natural form or leave a residue?

[fritchie]

Helenium - Many natural objects will burn out of a straightforward casting mold, so if you only want one cast, you can use the object itself. Tree trunks should work fine, if they aren’t too large and you can sacrifice the piece. If you have doubts, consult with a nearby founder if you can. I have seen crabs, fish, strawberries, oranges, pinecones, and so on, as well as wood cast that way.

Also, people swear by alginate, a seaweed extract, I think. It picks up exquisite detail, it is said, and typically is used on the human body, so should be quite clean. It's apparently also expensive. From my reply, it's clear I have no direct experience. Best wishes on this.

[obseq]

Originally posted by fritchie
Helenium - Many natural objects will burn out of a straightforward casting mold, so if you only want one cast, you can use the object itself. Tree trunks should work fine, if they aren’t too large and you can sacrifice the piece. If you have doubts, consult with a nearby founder if you can. I have seen crabs, fish, strawberries, oranges, pinecones, and so on, as well as wood cast that way.

Also, people swear by alginate, a seaweed extract, I think. It picks up exquisite detail, it is said, and typically is used on the human body, so should be quite clean. It's apparently also expensive. From my reply, it's clear I have no direct experience. Best wishes on this.

Alginate is a very good product. If I recall correctly, there are two types available and only one of them is optimal for casting--Unfortuantely I cannot remember the distinction.

Each package is about $15 and can go quite a long way. About 2 years ago I cast a full torso, two arms, and two legs with about $100 worth of alginate.

Since I bought in bulk I got a decent discount. Any dental supply store should have what you need.

Good luck!

[obseq]

Helen,


In case you are checking back, I have some more specific information regarding alginate.


The initial price of about $15 was off. It seems I was "ripped off" when I first made my bulk purchase about two years ago. All swindling aside, I paid $7 per one-pund package last week at a dental supply store.

Alginate comes in quick set and regular set. The quick set leaves you with about 90 seconds to apply the mixed alginate before it congeals and hardens. Obviously this is better for someone with perhaps experience with the product. I opted for the regular set at about a three-minute window of working time.

Hope this helps!


Good luck.

[jfmenna]

If you don't want to harm or contaminate your pattern you should check out some silicon press-on mold making material like the kinds produced by Smooth-On. I think if you called them and described your mold making needs they'd set you right up.

[obseq]

Helen, or anyone else interested.


I did some casting in alginate last night and fell upon a few old suprises along the way.


Frist, be sure to underestimate the working time before the alginate congeals. This began to occur almost immediately after I poured.

Second, just bypass mixing by hand. You wont be fast enough no matter how many eggs you've had the pleasure of whisking Instead opt for nothing else but an electric drill with a mixing paddle until smooth. It is best to have someone helping you emulsify the powder with the water while you mix.

Finally, if you are supplementing the alginate with plaster bandages as I did, have them ready to go as soon as the pour is complete, otherwise the bandages will not adhere to the alginate properly and you will be left with a partially seperated mold as I was last night.



Trial and error.........good ol' trial and error.

[ArbitraryDesign]

Originally posted by obseq
Frist, be sure to underestimate the working time before the alginate congeals. This began to occur almost immediately after I poured.

The working time can be increased somewhat by using water that is just above freezing in temperature. Likewise, hot water will speed up the setting time.



Robert

[obseq]

Originally posted by ArbitraryDesign
The working time can be increased somewhat by using water that is just above freezing in temperature. Likewise, hot water will speed up the setting time.



Robert

Great tip!

Thanks, Robert..

I am going to give it another try this weekend--I'll post my results once again. In the meantime I will be crossing my fingers.

[anne (bxl)]

The working time can be increased somewhat by using water that is just above freezing in temperature. Likewise, hot water will speed up the setting time.


same for plaster....

[obseq]

Can either of you give any advice with regard to ideal water-to- solid ratios for alginate and plaster?


Thanks in advance..!

[anne (bxl)]

an ideal ratio ? probably close to 1/1 but I always have had work feeling the material rather than calculate. here is the way I proceed :

use a souple plastic recipient (easier to clean up afterwards)
put water till mid height
spread up plaster by small quantities (just like rain)
slowly so to avoid lumps
add plaster till it comes to the water level (plaster as to appear like a small iceberg in the ocean)
wait for the plaster getting wet
start to mix up, preferably with fingers, you'll get a more creamy substance.
mix slowly to avoid bubbles.

... enjoy your experience!

anne.

[ArbitraryDesign]

I'm afraid it has been quite some time since I have worked with either plaster or alginate.
And when I did work with them, I usually just mixed them by eye.

As anne said in her post, I poured the dry material into the water until it formed a small "iceberg", or mound in the center of the water, and then added material as needed.
I usually tried for a consistency like thick pancake batter.

One thing you may want to look into is Hydrostone or Dentstone instead of plaster.
Plaster tends to shrink and distort, also has a chalky consistency that I don't like.

Both Hydrostone and Dentstone are gypsum cements, and are less prone to distortion and shrinkage. Dentstone is the higher grade of the two.
You can find Hydrostone (and maybe Dentstone) at many art suppliers, but it can be found for less money at Dental supply houses.

Robert

[darhorn]

Douglas and Sturgess (415) 896-6283 has a product called Moulage that can be reused after casting. You need to heat it up then let it cool to about 100 degrees if you are using it on living surfaces. I just got a batch but have not had time to use it yet. The company claims that Moulage is used by crime labs for evidence recovery.

Dar

[Helenium]

Re the side discussion about plaster mixing:

I only use plaster for making mother molds these days (to support polyurethane molds I mix el cheapo plaster with hemp and pat it on the polyurethane mold to make a shell). For plaster mixing the basic ratio of 2.5 lbs. of plaster to 1 quart of water is a good place to start. I've used Hydrocal for casts in the past (but don't anymore, for many reasons), but with that you generally use a little less water -- about 24 oz. Depending on the air temperature the measurements will vary. What you're looking for is a mixture the consistency of heavy cream, and you don't want to start with too much water or the plaster will be crumbly when it dries. "Mold Making, Casting and Patina for the Student Sculptor" by Bruner Felton Barrie covers plaster mixing and casting in detail, with lots of photos, which I found helpful when I was getting started with casting. The appendix has some useful charts in terms of planning how much material you'll need. I'm pretty sure it's still in print.

Re the responses re alginate, etc.:

Very interesting; many thanks to everyone who has responded so far. The sense I'm getting from what I've read here so far is that alginate is okay for small things, but not for anything you can't cover in a minute or two.

--Helen

[obseq]

<<Very interesting; many thanks to everyone who has responded so far. The sense I'm getting from what I've read here so far is that alginate is okay for small things, but not for anything you can't cover in a minute or two.>>


Helen,

The first time I worked with alginate was about 2 years ago when I had my arms, legs, and torso cast. This being a large area as a group, I am assuming that greater care was taken with the alginate than I did.

I am going to keep looking into some addtional information, because the material is relatively cheap and yields impressive results.

I'll be sure to post anything I find.

[jwebb]

I have to get my two cents worth in on this old but very interesting thread. Anne's description of mixing plaster, above, is beautifully said, and right on, in my experience. One of my mentors was European educated and apprenticed in France in early 20th century. He told us that he was only allowed to "mix" plaster, not pour it or do anything at all with it, for TWO YEARS! And watching him mix a pan of plaster showed it to be a minor art form in itself. He loved all materials so. Plaster was used on all those crazy fancy ceilings and moldings, not just sculpture then. Anyway, the only thing I'd add to the above is that it's often a good idea to mix a small, very thin batch of it first; pour this on to establish fine surface detail; then make thicker batches to add for "backup" to that. As for other molding materials, the medical and dental professions today have a wide variety of rubber-like synthetic casting materials that give excellent detail. A fellow student once got some latexlike stuff from a "plastic surgeon", and used it to make a mold of his own face. He then made a wax in that mold, to be used in bronze casting. After about a week, he looked at the latex mold and realized it had shrunken considerably. So he made another, smaller wax from it. A week later it was much smaller yet, and he made another wax. In the end he had this series of bronzes of his own mug, ranging from life-size down to a veritable shrunken head no bigger than an apple. Quite astonishing to see them displayed, all looking at each other.

[k10whosun]

Please let me know how it works out for you I would love to put the instructions at this site so others can try it out/

www.plasterlinks.com

[dondougan]

Hi all, jwebb & k10whosun -

I used a hydro-colloidal silica mold material (very similar to alginate in working) to cast a series of molds from very delicate antique alabaster carved sculptures. The hydro-colloidal silica mold material is very fragile (think Jell-O), and it starts shrinking as soon as the added water begins to dry-out. If casting plaster/wax immediately the casting will be 100% of actual-model size. However, if the mold is exposed to air and allowed to dry it will both shrink and harden, becoming less flexible at each degree. On one experiment, I allowed it to shrink about 30% (about 24-hours or so) and cast a smaller version in wax so I had both full-size and reduced-scale versions of the elements I molded. The 30% mold was about the consistency of (dry) cheddar cheese. I must note here that in the drying stages I had to clamp the mold in an adjustable-flask/form as it was drying and adjust periodically to prevent gross-distortion of the mold as it dried/hardened.

The only times I used it on porous material (Indiana limestone), I took no chances with porosity and sealed the limestone by using a torch and paraffin wax to fill the surface pores on the limestone. I have found the hydro-colloidal silica to be good for a very limited number of castings — the most I've ever successfully taken have been three waxes or three plasters before the mold was damaged beyond re-use. Though flexible for undercuts, it tears very easily. The advantages are that little (if any) separator is needed to remove the form from the model — for living skin certainly none is necessary . . . but then I've never tried it on bark. <grin>

Don
www.dondougan.com

[drcrash]

Originally Posted by dondougan

Hi all, jwebb & k10whosun -

I used a hydro-colloidal silica mold material [...] The hydro-colloidal silica mold material is very fragile (think Jell-O), and it starts shrinking as soon as the added water begins to dry-out. If casting plaster/wax immediately the casting will be 100% of actual-model size. However, if the mold is exposed to air and allowed to dry it will both shrink and harden, becoming less flexible at each degree. On one experiment, I allowed it to shrink about 30% (about 24-hours or so) and cast a smaller version in wax so I had both full-size and reduced-scale versions of the elements I molded. The 30% mold was about the consistency of (dry) cheddar cheese.

Interesting. I've been experimenting with homemade agar agar moulage, which used to be common for lifecasting and is still sometimes used in biology and forensic medicine for taking impressions of this and that.

One nice thing about agar moulage for general moldmaking (but maybe not shrinking things) is that it's reusable. You can melt it down, add a little water to restore whatever has been lost to evaporation, and make a new mold with it. My understanding is that you can reuse it 20 or more times. (Some people say 50 to 100 times, but I'm skeptical; I haven't gotten that far yet.)

You can also use gelatin to make molds, and it's reusable too, but not as many times. (5 to 15 times, according to some things I've read.)

You can make your own agar moulage relatively cheaply (compared to silicone, anyway). A few dollars worth of agar and a few dollars worth of glycerin or sorbitol and a few other minor ingredients (plus water) will get you a half gallon of the stuff.

You can make the shrinkage problem mostly go away by replacing essentially all of the water with glycerin or sorbitol. That increases the price several times, but since the stuff is reusable, it may be worth it. (Glycerin and sorbitol cost about 50 cents an ounce where I've been buying them---Hobby Lobby and a drugstore, respectively.)

Allegedly you can use gelatin-sorbitol molds for dozens of casts before melting them down and remaking them, but I assume that's for easy shapes with little or no undercutting.

(Gelatin molds used to be common in foundries for making wax plugs for lost-wax casting; I don't know if they still are.)

I did shrinking experiment recently with an agar moulage I made, reducing a face cast by a factor of about 60 percent by volume. (With mostly water and some glycerin.) It came out reasonably well, I think, but with some residual distortion. (The nose being a little too small and thin, for example.)

Then I read something in an old moldmaking book saying that agar in a sorbitol solution will shrink without distorting; I intend to try that next.

I'm hoping that by varying the ratio of water to everything else (controlling it by how much sorbitol I use for the "everything else") I can control the final size reasonably well, to scale sculptures down pretty close to a desired target size.

[Landseer]

You can make your own agar moulage relatively cheaply (compared to silicone, anyway). A few dollars worth of agar and a few dollars worth of glycerin or sorbitol and a few other minor ingredients (plus water) will get you a half gallon of the stuff.

The issue is rarely how to get the CHEAPEST mold, but one that will LAST the longest, the idea of making molds is to use them, not replace them over and over! and aga, geleton, latex and others are temporary and degrade fast.

The most costly portion of mold making is your ~TIME~ not the materials!
A half gallon of QM140 silicone would run about $55 and it goes a long way.

If I spend half a day making a mold I want the best rubber there is that will last for years and hundreds of casts, because I sure as hell don't want to have to redo it again after 6 casts or something. It's not the molds that earn you money- they COST you money and time, it's the cast that make you the money, so 6 hours making a replacement mold to replace a crappy one made with cheap materials that didn't last to save $15 v/s using that same 6 hours to sculpt a new model, or cast 25 castings, which do you think is going to put the money in your bank account?

[drcrash]

Originally Posted by Landseer

The issue is rarely how to get the CHEAPEST mold, but one that will LAST the longest, the idea of making molds is to use them, not replace them over and over! and aga, geleton, latex and others are temporary and degrade fast.

For me, it's not rare at all to want a cheap mold I can use once, or maybe twice or three times. It's the usual thing.

Most of the molds I make are one-offs, which I use to cast a positive shape that I then re-sculpt and re-cast, and/or shrink, and/or vacuum form over to make the final object.

If I had to pay $100 a gallon, I just couldn't afford to do most of the things I do.

There are lots of situations where people don't take advantage of molding and casting as an intermediate step because they think it's too expensive. That's a shame.

Life casting is one example. If I can afford to make several lifecasts, I'm more likely to get a good one that turns out well. If I like it, I can sculpt the eyes open, modify the features, etc., and make another alginate or agar mold to cast the final work... or make a more permanent mold from silicone or urethane if I want to make a bunch of identical objects.

If my mold material is cheap enough, I can afford to do more experimentation before committing to a more expensive process, and I can "cut and paste" the parts I like to assemble something I like better.

There's also small-volume work where a temporary-but-not-too-temporary mold makes sense. For example, if you want to cast a few bronzes, or replicate some plaster trim for repairs, a gelatin/sorbitol mold may do the job just fine. If it turns out there's a market for a bunch of copies, you can make a silicone mold later. (As long as you don't damage the original making the initial temporary mold. Alginate or agar generally will not damage the original unless its intolerant of dampness, which is one reason they're commonly used in certain fields where you don't generally need to produce a bazillion copies.)

Silicone is great stuff, but there are many situations where it makes sense to use something cheaper. You don't want to pay that much to do something that's a one-off, or likely to be a mistake. (Or an incomplete success that needs some fixing up.)

Silicone is great for volume production, but cheap mold materials support a more exploratory kind of creative process. Molding and casting are useful for many things besides producing a bunch of identical copies from a final sculpt, for sale.

[racine]

ADDITIONAL NOTES ON PLASTER.
Annes method is the most practical for sculptural /moud making, [mixing by batch is slow and usually for ceramics moulding]however the ice berg is not required, just under the surface and level is a guide [this is the RCA foundry method] if u have sifted the fresh plaster well through your fingers there will be no lumps. use cold water.
agitate the plaster minimally if you want it to work longer, use a wire whisk which can be made easilly.[plaster handle]

if applied by soft brush and agitated on application the runny plaster will strenghten, when the bowl starts to thicken [wash the brush]use a spatula and leave a key for the next layer then wash the equipment. [not down the sink]

[racine]

notes on plaster[interrupted] continued;
dont put the stuff down the drain, if you need a simple method use an old bucket filled with water, wash off there and next day pour off the water and bin the settled gunk. [alternatively it can be thrown at the next door cat thats defecating in your radishes , all cats are grey and blotchy in the dark???].

plaster[of paris] types vary, the softer ones expand a few percent and anything under 0.3 percent is possible to make good molds with, as a rule the harder the plaster the greater the expansion. but a basic 'dental' plaster or a slightly better fine casting plaster can increase amazingly in strength by agitating it when applying, eg brush and spatula etc, but dont overwork [killing the plaster] or re water as this has an opposite effect. when its warm scraping off only.
scrim [hessian, a nasty fibrous medieval sacking material] is great as a secondary strengthener for moulds up to 2'. after that u need steel as well.

[purplecat]

Some of this has been of great help to me, thanks!