This is a very basic how to guide for casting a model up from a print. It’s very easy to follow and works well with a variety of prints. All of the materials are easily obtainable from any high street, and at the very worst, a quick click away from a website.
It’s a very good idea to try this method out a few times at home so you can see how easy it is, and to familiarise yourself with how the plaster behaves. The main fear or worry with working with plaster is it suddenly setting on you, so if you know what to expect, it will become easier with each casting.
What You Will Need.
Polythene mixing bowl
Plaster of Paris
Absorbent kitchen towel
1-3 small bottles of tap water
An old trowel or butter knife
An old nailbrush
Plastic bags ( carrier or bin liners )
1-3 long screws or hooks
Blue or red food colouring
Step 1. Fill your bowl about half full with water. As a general rule of thumb, by the time you’ve added your plaster to the water, the amount in the bowl should be just under double the amount of water. Always mix a bit more than you think is needed, until you become more used to quantities.
Step 2. Give the opened plaster bag a quick stir with the dry spatula. Most commercial plasters are well processed, but lumps can occur during storage, so break up any lumps in the bag.
Step 3. Slowly shake the plaster bag over the bowl, filling the bowl until you start to cover the water over. Little peaks will start to build up and come clear of the water, so keep going until the water is covered over. After 30 seconds or so, little cracks should form on the surface, like a dried out lake bed PIC 2. Use the spatula to move any dry plaster into any wet areas. If it’s really wet, add some more plaster until you get the look of this consistency
Step 4. Once you have the dry lake bed look, leave everything to soak for 2 minutes. This is easy with a phone, and allows you to check the print you’re going to pour into. Remove any sticks or leaf litter from the print, and use the absorbent towel to soak up any excess water from the print.
Step 5. When the phone pings 2 minutes, it’s time to stir and pour. Slowly mix the plaster well for about a minute, again using the phone if you’re hopeless with timings. Scrape the base and sides of the bowl with the spatula to catch any pockets of dry plaster. The main aim is to break up any lumps and mix slowly to avoid whipping air into the plaster, minimising any air bubbles.
Step 6. the ideal consistency to aim for is between a pourable yogurt or a thick pourable sauce. If it’s like mayonnaise, it’s OK to use, just a thicker mix. Slowly pour the plaster into the print, using the spatula where needed to help move the plaster into the print.
Fill the print up level to the sides, and use the spatula to help even everything out.
Step 7. As a rough guide, you have about 8-10 Min’s working time with the remaining plaster, so slowly stir what remains in preparation for building up the rest of the cast. This is just like icing a cake, as much fun and no more difficult.
Step 8. Periodically stir the plaster, and you’ll feel the mix slowly firming up. As it does, start scooping it onto the cast building up from the front of the impression towards the back.
By the time you reach the back, the plaster will have firmed up for another layer. Give the remainder a good mix, and continue again from the front to the back. Keep in mind that the arch of the foot area, will be the thinnest part of your casting, so be sure to make that part nice and solid. Before the plaster gets too hard or firm to work, now is the time to sink in your long screws or hooks, but don’t go too deep.
Step 9. When you run out of plaster, sloosh some water around your bowl and spatula to help reduce clean up, and continue smoothing out the surface of the cast using the spatula. When the surface of the plaster starts to harden, you’re out of time, so try to make things as neat as you can.
Step 10. Now you wait. Most plasters claim to be ready in about 20 Min’s, and in a mould at room temperature, that’s about right. But outdoors, temperature and moisture can affect setting times considerably. To avoid any cracking, crumbling, or loud swearing, allow everything to sit for a good 30 minutes to set, 45 if it’s wet or rainy. As plaster sets, it heats up, so don’t be too surprised if the cast feels warm to the touch.
Step 11. Dig things out, and congratulations! You’ve just cast your first print. Use the butter knife or trowel to dig away the soil at the edges of the cast, and if you inserted hooks or screws, use those to help lift the cast out. Have a good look and think about how you remove the casting: digging away at the edges may damage any nearby prints, so have a good look before deciding. By it’s nature, plaster will drag some soil out of the impression, no matter how carefully you handle things, so see how digging away at the edges works. Just go slowly and carefully and when you feel it move, lift the cast straight upward.
Step 12. Use the nailbrush to take off any excess soil and wrap the cast in a plastic bag to keep bag mess to a minimum. The remaining water/plaster mix is non harmful and can be poured away, it’s even good for your garden! As soon as you empty the bowl of plaster, fill it up and sloosh to remove the remaining wet plaster. Dry everything off, bag it all up and you are done.
Optional. In many dental laboratories, Plaster of Paris is the standard substance for non important models and moulds. Kaffir D stone is a slightly harder, yellower plaster that makes for a more durable model that will put up with several boiling processes, and as it behaves identically to Plaster of Paris, can be mixed 50/50 to make it go a bit further. Artificial stones and rocks are the most expensive of the casting substances, but yield an incredibly fine detailed and tough model. Their colour is usually a cobalt blue or salmon pink, and they are used for detailed crown and bridge work, where the tiniest details need to be seen. For a cheaper option, add several drops of food colouring to the water before you add the plaster and mix in. Any dermal ridges or finer details will be much easier to see with a coloured stone.
D. Davies 1/10/17