this is another awesome idea that someone else had before I did, here is the original instructable
I really liked the idea of this instructable but unfortunately I don't have a laser engraver or a 3D printer or a CNC router so I have a method for us mere mortals who can't afford or don't have this equipment but still want to create this kind of awesome project.
Step 1: What you will need
Plaster bandages, also known as mod rok or mod rock
Oil based clay or Modelling Clay, e.g. plastercine you could use playdough or salt dough
Plastic posts or old pens
Pot for measuring
something to mix with, I used a small trowel
and a veg patch to put your grave stones in ;-)
Step 2: Making your sculpt
just sculpt what you want your tomb stones to look like, I have done 2 so far, carrots and rhubarb
I will be doing some more for my herbs and other veg, and also a plaque for the whole veg patch.
I have shown my carrots stone with a ruler so you can get some idea of scale, I sculpted them on sheets of transparency which I had used a while ago for screen printing negatives which I no longer needed, you could just cover a book with a bin bag or a sheet of glass.
something I didn't think about when i created these was the edges you want to make sure the edges are as straight as possible you don't want them to taper in e.g.
/ \ like this is fine, it will come out of the mould properly
| | like this is perfect, it will come out of the mould properly
\ / like this is not ok it will get stuck in the mould when you cast, I found I had done this without realising.
you shouldn't worry too much if there are dips or nicks in the surface, this will make it look older when its cast.
Step 3: Making the latex mold
paint a layer of latex over the whole mould making sure you get in to all the details this is the most important coat.
leave this layer to set, it should go slightly transparent, then add another layer let it dry and over and over until the latex is thick enough, make sure you get a good coat on the edges.
once you have the thickness leave them for a day or 2 to dry out a little more. DO NOT pull at the latex or try and lift it off the surface you have put it on, it will still be damp inside and may tear.
Step 4: Plaster jacket
get a bowl of water ready, make sure you have plastic down or something on the table, I didn't because the table is old and I'm not too worried about it.
cut up your mod rok in to short strips, I used 2 rolls per tomb stone
take a strip of mod rok submerge it in the bowl of water remove it and while holding it over the bowl massage it to work the water in to the plaster in the bandage, then place over the latex mould and again massage it, put your strips horizontal to start with and vertical and alternate until the bandages are used up, make sure you leave enough around the edges around 3cm should be enough.
once you have finished you need to leave them to dry, these ones took a while to dry because the mod rok was old, leave them for a few days in some where warm like an airing cupboard.
tempting as it may be DO NOT put them in the oven, latex will burn, and the plaster might crack up.
Step 5: Prep the mold
make sure you get all the plasticine out of the details you may want to use a pin
due to the difficulty of removing the sculpt I knew I would have trouble when it came to removing the cast, this is because I messed up the edges as discussed in the previous steps.
to resolve this trouble I decided to split the jacket, using a dremmel and a cutting disc I cut most of the way through so I could snap the jacket open later once the casting was dry. if you got your edges right you won't need to do this.
I also trimmed up the edges of the latex mould and the plaster mould with scissors
now you need to leave the latex mould to cure all the way through this might take a few days it will turn a yellow colour.
Step 6: Casting
now you need to mix up a little of the concrete mix just get a small amount and put it in the jug, we need to make this one very wet, I call it the beauty coat concrete users will call it a slurry mix the consistency of soup if you can try and keep as many stone out of it as possible, the purpose is to get concrete in to the details of the mould. you only need a little in the mould to cover the bottom, then agitate it to get it in to all the cracks of the mould.
now you need to add more concrete mix to what you have in the jug you need to make it a thicker mix your after a mix that's the consistency of cake mix, spoon it in to the mould and tamp it down to get it all in the mould this also helps to bring the fats from the concrete to the surface and the excess water to the top.
now we have to insert the plastic rods, you can use old pens but I would remove the ink bit from inside you don't want this in your soil.
ok now you need to leave them for a minimum of 24 hours, I would leave them a few days to make sure they cure all the way through.
Step 7: De-mold and tidy up
Now as I have already cut most of the way through the jacket all i have to do is push the jacket backwards and snap along the cut i have already made and separate the jacket.
using your fingers rub off the excess concrete off the edge of the mould and break off any thin bits that have leaked out of the mould.
peal back the edges of the latex mould turning it inside out, then peel back the latex mould from the face of the tomb stone and you should have a perfect copy.
now all you need to do is break any thin bits off the back, if you wanted to you could use a file to tidy up the back edge.
if you wanted to make more copies all you have to do is hold the jacket together with elastic bands or tape.