What is the best concrete to use for casting?

[ThrowingChicken]

I've been searching the net and haven't found much info, and I myself know very little about it. I bought a big bag of mortar but the issue with that is when I rub my finger along the cured surface little bits of the mortar rub off with it as if I were rubbing dried mud. If Andy Dufresne's cell were made out of this stuff I doubt it would have taken him 20 years to escape.

Let's just say that I want to cast something that is roughly 12" in diameter and 2" thick, and something with a smooth consistency and won't break down so easy like the mortar would be idea; what's my best bet?
It was considering cutting it with some plaster, any thoughts on that?

 

[exoray]

Go pick up a bag of Portland cement (not concrete) Portland cement is just the powdered gray part no rocks or sand in it... It's not hard to find, and everywhere that carries concrete should carry it... Also pick up some fine sand, not the cheap traction sand that has large chunks in it, you want the kids play sand that is nice and smooth...

The mix ratio can vary but 1 part cement to 2 (or 3) parts sand, is a good starting point... Play around a bit with the mix ratios, less sand will be more like plaster, glossy smooth and pretty brittle and flaky and too much sand will get you to the mortar consistency you have experienced already... BTW mortar is simply Portland cement with a high sand and lime dust content... You should be able to find a happy medium around the 1:2 or 1:3 mix ratio...

The 1 part Portland to 2 (or 3) parts sand will be close to your normal concrete consistency, smooth but not brittle...

If you need more strength, you can get some fiberglass flock or strands and mix that in as well...

*Edit mix by volume not weight...

 

[ThrowingChicken]

Thanks that helps! Since I already have this mortar and it is as you said just Portland cement + sand, would it be safe to assume I could combine the mortar with straight cement and get similar results?

 

[exoray]

Thanks that helps! Since I already have this mortar and it is as you said just Portland cement + sand, would it be safe to assume I could combine the mortar with straight cement and get similar results?

Trash the mortar, the lime dust isn't what you want... You will have a much better experience with simple sand and Portland cement mixes...

 

[ThrowingChicken]

Okay, thanks for the tips!

 

[dropshipbob]

Years and years ago when I was in college, I worked at a place that made concrete lawn ornaments.

They used off the shelf bagged concrete mix and cut it with small gravel. If they had any holes or other blemishes that needed to be repaired, they used mortar.

Is the item you are going to make going to be inside or outside?

 

[qcfoundry]

i would stay clear of the concrete period...especially for something this small.
try a product called durham's rock hard water putty. imagine a product that mixes like plaster, but have the finish characteristics of a material thats 25% plaster & 75% resin.

its non toxic, easy to mix, pours/casts pretty thin (think a thinned out milkshake), cures in 24 hours, picks up great detail on the mold, is easy cut and drilled (doesn't crack) and even sharp corners won't break off too easily.

its about $10 for a 4lb box.

 

[ThrowingChicken]

I did a mixture of Portland cement and sand as Exoray recommended and so far it seems better than the mortar, though it hasn't fully cured yet so it is too soon to tell. Right now I am just making the bucks, the first of which I used mortar to shape a rock texture and I could tell then that it wouldn't work well for the casting process. It wasn't that great for the shaping process either. When it comes to actual casting I may try that Durham's stuff, and I'm also looking into mixing ground limestone with resin as a gel coat then reinforcing the back with fiberglass or foam to make a light weight rock looking item.

Here is what I am working on if anyone is interested:

 

[exoray]

mixing ground limestone with resin as a gel coat

Don't use the commonly found 'hydrated' limestone!!! It will erupt like a volcano when you mix it with resin, same can be said for even plain limestone that has been laying around as it absorbs water...

Also if you are going to play with resin...

Your best bet would be to get some ground (gray) ceramic (porcelain) dust, Smooth sells it as URE-FIL 3, but you can get it cheaper from a ceramic supply company or a local industrial resin supplier... When I say cheaper I mean CHEAPER! Call some ceramic supply houses and inquire about their fillers...

The only real drawback is that no matter how much of the ceramic you add, it will still resemble resin to some degree... But like cold cast metals, if you work the surface you can bring out more of the filer properties... Say for example a little sanding or roughing up with sand paper, it will make it feel more like stone and supress the plastic like feeling...

 

[PTK]

if you want good detail and strength i'd suggest Ultracal 30

if you don't care about the color and can spend a little more money go with Die Keen green dental stone.....very strong, chip resistant, great detail, maintains size........all around great stuff

regardless of what you use I would definitely throw in some Acryl 60 with your water.

a few other things i've done that seem to help are use water around 100 degrees F. If its really important use distilled water, but in most cases it shouldn't matter. You can also soak some burlap or other reinforcement material in there for strength and to prevent cracks. I also like to cover the piece with plastic sheet or garbage bag as the material starts to heat up and steam, it seems to help form a harder, stone that is less likely to get flakey on the surface.


hope that helps!

 

[PTK]

oh, and if you wanted to go the fiberglass route there is some stuff called FGR 95 made by hydrocal......it is a plaster made to be used in conjunction with fiberglass.......brush your detail coat then back it with more fgr and fiberglass mat for super strong castings......but i think this would be overkill for a smaller piece

 

[dropshipbob]

Hey, I LIKE THAT!:thumbsup Sort of a Matrix sentinel fossill kinda thing?
Again, is this going to be outside?
Are you going to make copies available?:cool

I had something like this in mind when I worked at the concrete ornament factory, and the owner was all for me making something original that he could sell, I just didn't have much time or the knowledge to do so at the time.

I did a mixture of Portland cement and sand as Exoray recommended and so far it seems better than the mortar, though it hasn't fully cured yet so it is too soon to tell. Right now I am just making the bucks, the first of which I used mortar to shape a rock texture and I could tell then that it wouldn't work well for the casting process. It wasn't that great for the shaping process either. When it comes to actual casting I may try that Durham's stuff, and I'm also looking into mixing ground limestone with resin as a gel coat then reinforcing the back with fiberglass or foam to make a light weight rock looking item.

Here is what I am working on if anyone is interested:

 

[Contec]

What is it ?

 

[ThrowingChicken]

Hey, I LIKE THAT!:thumbsup Sort of a Matrix sentinel fossill kinda thing?
Again, is this going to be outside?
Are you going to make copies available?:cool

I had something like this in mind when I worked at the concrete ornament factory, and the owner was all for me making something original that he could sell, I just didn't have much time or the knowledge to do so at the time.

It's a robotic squid. I'm doing a series of fossilized robotic animals; cat, squid, bird, turtle, and human. The concept is, "What will they be digging up a million years from now", but I wanted to take it further than just present-day items. I had actually wanted to make a full scale human robots, but I didn't feel that I had the funds or the space accomplish that goal, so this started off as a stepping stone.

The series will hang in a local gallery in May, after that who knows. Hopefully they have some appeal and I can sell a few casts to recoup my costs, and if I'm blessed, my time. The final pieces will hopefully be cast out of a light weight material that resembles stone. What that material will be I do not yet know, but when it comes closer to that point I'll probably make a proper thread and see if any of the knowledgeable people here have any ideas.

 

[androidandy]

"Future Artifact". I have always loved this concept. Very well done and very cool. Have you thought about using traditional sculpting materials for the original to make a mold from and just cast the copies in the concrete, or am I missing the point completely I think I may be interested in one of these when you are done.

Andy

 

[ThrowingChicken]

I guess I just figured concrete would be the quick and easy. I just build a little support system that holds up all of the pieces to the level I want them to be exposed, add a bounding box and fill it up and the concrete just gets in all the little areas I need it to go. It's not so bad, even the first one with the mortar isn't too bad, but if there was a better method out there I wanted to use it.

 

[dropshipbob]

Throwing Chicken, I love this concept! And I hope that you post pictures of the whole line when you have them finished.

If you want a material to make copies in later that's lightweight and cheap, plaster should work. I made a few sculptures using alginate as a mold and plaster as the casting medium and I was going for a stone-like texture. I was pretty happy with how they turned out. And if you want a bit of texture, you can always use some of that texture stuff that painters add to acrylic paint. I've used that too with good results.