A lot depends on what type of finish you are going for.
A Portlands/admix slurry will give you a very smooth finish.

I would experiment with small amounts of slurry and dry pack in a corner of your mold until you have a finish that suits you. A word of warning however, you can still end up with invisible voids BEHIND the slurry coat which could cause the slurry to break away long after the piece is finished.

Thanks Tufaenough. Of course!, if I'm getting voids between the dry pack and the inside of the mold, then I could still get voids between the drypack and the slurry. The problem might be a Tamping issue.I'll work on that, and see if I can get the desired finish, without any voids under the slurry coat.
Forgot to mention that the inside of the mold is textured, that's why I intended to use a face coat, to capture the detail. And I want to mix colour into the face coat, to save having to use large amounts thru the whole mix.
Thanks again Tufaenough, started the ideas flowing again,
Slate

Hi Slate
I don't cast much at all because I have no molds.
I'm hoping to make my own molds soon enough. Simple stuff.
I'm working on a 3 part perlite mold of an old work boot.
Undercuts will have to be avoided and cut into my casting after unmolding.

But I think the key to avoiding voids is good vibration.
Even 'dry' tufa liquifies under good vibration.
I've developed a nice vibration table that rests on my air compressor but haven't used it much.
I'm still doing simple outside casting for my pots, stones and masks. I like the control it gives me over the finish.

Slate-for fine detail,recommend using a sand/perlite combo with your cement.I'm doing a mask that's about 26 inches across and 3-5 inches deep.Using a standard sand/cement mix,this sucker would weigh,oh,about 70 lbs or so.I've got it down to about 35 lbs now.Still heavy,but I can carry it.No hernia here!!

Hi Tufaenough,
I'm wondering what material your going to make the mold out of, for the boot. Latex? I was a bit disappointed when my lated mold shrunk. I went to a Smooth-on seminar here in Oz, a few wks ago, and they told me the latex can shrink up to 15%. The shrinkage in mine makes it difficult to work with, as it doesn't sit snug up against the fibre-glass mother mold. That's why I went to the seminar, to check out other mold materials. I was impressed with the silicone rubbers, but they are expensive, and I'm just starting out. So I'll make do with latex till I get better at this.
Almost forgot, what will you use as a backup mold?
And a release agent? Let us know how you go with the project.
Adios,
slate

Rick do you have to fill the mold with cement?
Can't you use a thin coat like leaf casting.
I do my masks on the outside of my form ( they don't have the detail of the cast masks but I like them) so they are hollow once the form is removed.

This is probably a really dumb question but aren't real Moai heads carved from lava rock and have a really pitted appearance? Wouldn't one want voids on the surface?

Deb

Slate I'm trying something new, at least I've never read about it. My mold won't have the durability of latex but it should be endlessly repairable
The boot has to be carefully prepared to avoid under cuts so I'm filling spaces and covering the boot with wax.
It's not a priority project so I might not finish until this fall.

If this works I'll let you know.

Hi Rickharmer,
can you tell me what ratios perlite, sand, cement? I tried perlite once, and it turned into slush. Maybe I over-mixed it. As for the weight, if I find my statue only half way down the drive one morning, I'll know why!
Debzone8, yep, I want a textured/pitted appearance, but the voids I had were obviously mistakes in the casting, and wouldn't double for weathered stone.
And Tufaenough, I tried to do a 'hand lay-up' casting of the piece, but because of the inward falling angles and undercuts of the mold, the lay-up just fell back into the bottom of the mold, no matter how thick or dry I made the mix.

I am curious why if the mask or piece is hung on a wall why it would not be hollowed out in the back, or after packing/patting your mix in you could not put a piece of stryofoam in and then continue to pack around that if you want it to look solid.
BTW most molds can take a fair amount of packing, I would suggest that some of the voids are caused more by not packing/generously patting the mix to the point of having a 'slurry' flow into the texture lines.
Dena
Dena

Hi Dena,
thanks for the input on foam insert in back of a wall-hanging, but this piece is a free standing statue, just like a real Moai statue. It's a couple of feet high, and one foot wide by one foot deep. So when pouring the cast, it's like filling a deep hole, completely surrounded by the mold, and with other little voids going off to the side, which are the cavities for the nose and jaw.
Sorry, I didn't explain myself properly.
Thanks,
Slate

Hi,there.How's your weather down there?
To mixes:I've been using 1:3 for some time now,both in hypertufa,and again in my other mix.
Basically,1 cement,1 sand,2 perlite,to reduce weight.Appreciate more could done(as Dena suggested)but I'm filling them up,knocking them out a few hours later(more like GENTLY sliding them out!).I'll also use admix,and plasticizer(about an ounce for my mixes)and more recently(thanks to posters here)coating the mask with a cement"paint"to lessen voids.That works for me.I've had a 35 lb mask hanging up for a month,no cracks or stress shown.

Thanks Ric,
for the mix to try. My first statue probably weighs about 100 lbs. This should lighten it up considerably.
As for the weather, it's the first month of winter here, and day temps of 18degC. Can't wait till Spring!
The perlite doesn't show thru the outer cement face coat?
Sorry for the basic questions everybody, but I'm a Newbie, and not familiar with this process.
Slate

With a "paint job"there's no perlite showing,plus if you can use the finest grade,that helps too.
And,its about 13 degrees C here and raining.

Rick how do you the 'cement paint' isn't just hiding voids behind it that will break out later?

Good question,for which I hope someone else has the answer.Remember that this paint is to take care of very small holes,not larger voids,those would be taken care of by vibration and good mixing.I always give my mixing tub a few thumps to take out the air bubbles.And,perhaps,I use a wetter mix,as I've read about in other forums.This will if put in slowly and at one point, further lessen the nasty voids.Unless you like that weathered,pockmarked look that good old pollution can give to fine statuary.
And...
Hands up those of you practicing DARK SKY in your community.Do you show up on satellite photos?We don't.Our sky at night has zero light pollution,and is very restful.So that I can wake up ready to mud again another day!!!
Cheers

Slate1,
I am assuming the mold is standing on it's head, and once done you turn it over and stand it up?
I still think you could lighten it up even more than perlite. If the mold is on it's head (empty) start by putting in some mix and tamping it down, then insert a piece of PVC pipe, and position wires (like croquet hooks) over the PVC and the outer edge of the mold. The hooks are to hold the PVC in place, you only need a couple. Then continue to fill your mold with the mix. You can still tamp your mix into the form or gently vibrating/tapping the outside will bring your slurry to the surface filling the voids that you were concerned about earlier.
I would cut the PVC about 2-3" shorter than the height of the mold. The hooks are only there to stabilize the PVC as it will want to rise up a bit.
Dena

Good advice,Dena.Sorta like doing columns.
Cheers

rickharmer, I would LOVE to be able to put my hand up and say I'm practicing DARK SKY, but alas, I'm in Calgary, which is apparently one of the worst light polluters in North America. :( We're working on it though, changing out street lights and such. I grew up on a farm and truly miss being able to actually see STARS!!! I seriously envy you and and your location.

~GC

Dark skies are cool. I have very dark skies and get to put my 10" dob telescope to good use at least once a week.
Except lately...rain rain rain.

Ric, thanks for the info on cement paint & perlite.

Dena, you obviously thought a bit about this. I made a couple other inquiries, and heard the insert called a 'plug'. I wouldn't have thought to stabilise the PVC with hooks or straps, though.
As soon as I'm able, I'm going to try it out. Had an accident, and can't put any weight on the foot, which means hard to work in the garage. Oh well.
Thanks again for the info, when I get one poured successfully, I'll try and borrow a digi, and post a pic.
Slate1

Slate1,
When my son and I made the columns for our yard, I encountered a 'floating' pvc insert, and had to start over. We duct taped the PVC on the end that was on the ground (while putting the mix in) and then my son came up with the wire (we used coat hanger) to stabilize the pvc and allow for it to maintain a sort of central position. It worked quite well. With the stablizers we would have been able to hold a piece of pvc pipe that was shorter or longer than the outer form; yours is the latex mold ours was a rigid sono tube form. Making the inner piece longer would have allowed for a shorter column, pedestal to be stabilized in ground later by filling that extended area with concrete to stop those pesky squirrels or moles from tipping something just enough off balance. Hope to see what you come up with.
Dena

Dena,Gotcha, but a question. Did you leave the PVC pipe in the column after it was set? Did you use a runny mix to slip easily all the way to the bottom of the form/mold?
Did you tamp the mix as you went? Did the finished product show any voids, or visible joints, like layers, on the outside of the finished piece?
Sorry the Q&A format! You've got me interested in doing columns, Ancient look, of course. They would go well with the primitive stone statue theme.
I'll try and find pics of your columns in here,
Thanks so much Dena, you've been a big help to someone new at this.
Slate1

Yes we left the PVC insert in the middle, I think it actually provided some extra structural integrity overall.

I used a concrete mix, regular ole stuff you can buy a big hardware type store, I see you are in Australia. I then added sand and perlite dry colorant and fibers (an additive that can be gotten on line or perhaps if you ask at a redi-mix concrete store they will sell you some). I think if I did it again I would not add the fibers, just don't think they were necessary. To each bag of concrete mix (60lb bags) I added about 4cups of sand,4cups of perlite and a small handful of fibers. For the 48"x8" sono tube with a 4" PVC pipe in the middle I think I used approx 3 60# bags, I think I figured I could have used 2 80# bags instead with all the additives that I did; that part was actually sort of easy to figure if you look at the bag. I mixed mine in a cement mixer (also not necessary but convenient, sort of) to a fairly dry consistency. The first colum I made the mix too runny and there is a difference when I pulled the sono tube away. anyhow back to what I did. The drier mix I dropped in beside the PVC pipe and initially tried to go around fairly evenly so I did not have to adjust the PVC any more than I'd have to, the clips were to hold it but it did move slightly. By dropping the mix in and not tamping, the columns were mostly rough with spots that looked like they had eroded.The weight of each drop of mix ontop of the other actually did it's own settling. A few times we used a 2x4 to sort of tamp things down, but we did not want it solid.
Things I would do differently is:
1)mix would be more consistent and drier, I stayed with the ingredients and that worked well, but had to figure out how much water worked in the mixer.
2) do the midcolumn section in shorter pieces, with couplings to fit them all together, 48x8" weighs a lot. sections of 16" would be much more reasonable.
3)release the column from the sono tube earlier and rough up the lines that were left, I still do not really like those.
4) use more colorant, and even consider each batch (or different bag) be a different color so it would look like stacked earth
We altered the way we set them up, after the first season, instead of rebar into the ground we placed a reducing PVC coupling piece into the bottom that fit into a cement filled plastic bucket dug into the ground. We did this after one of the columns with just a flower bowl on top, fell over in the winter, the column just missed a weeping cherry and only broke the toe on Lizzy (4ft concrete lizard)glad no one was hurt, lesson learned.
Here is a link to my webshots where you can see the columns
I am very pleased overall and believe it is a doable project
Dena

Here is a link that might be useful: columns/behind the fence

Dena, thanks for the detailed info on pouring columns. This kind of how-to info is invaluable. Impressed with your columns!
Slate