Utility plaster from retail building supply shops, art shop etc. Shop around as prices can vary a lot.
Re-cycle old plaster:
You can even recycle your old plaster castings. Smash the old plaster to a powder with a hammer. Work on a concrete
step - not on the kitchen floor! Try to keep the pile of powder from spreading out too much. The more you smash, the
finer the powder. Heat your gypsum powder in the oven to 160°C for 40 minutes. Let it cool and see if it sets when you
add water?
Make plaster casts:
| When mixed with water 'Plaster of Paris' hardens and then becomes hot. DO NOT attempt to make a cast enclosing any
part of the body with 'Plaster of Paris'. Failure to follow these instructions may cause severe burns that may require
surgical removal of the affected tissue. |
Plaster of Paris is great for making plaster casts!
Getting started: You can buy a plaster-casting kitset from a toyshop for under $10. The kitset usually includes a small
pack of plaster of Paris and a rubber or plastic mould. You mix up the plaster with water and pour it into the mould
and 1 hour later you have a plaster model that you can paint! You can re-use the mould many times and when you run
out of plaster of Paris you can buy more from a hardware shop.
An idea: Around Christmas time you can 'go into production' and make plaster models for all you uncles and aunts!
A tip: When mixing plaster of Paris, ALWAYS add plaster to water. NEVER add water to plaster. If you want 2 cupfuls of
plaster, then add about 1 cupful of water to an old ice-cream container. Then, sprinkle dry plaster into the water by
hand. Add the plaster more slowly as the level of wet plaster comes JUST to the water surface. Now stop - this is enough
plaster! Let it stand for about a minute then stir it smoothly for about a minute to include any small clumps of unmixed
powder. If you stir too hard you get air bubbles into the mixture.
A tip: You can speed the setting of plaster of Paris by adding some gypsum and you can slow it by adding a few
drops of lemon juice. Don't add too much or it will take ages!
A tip: You can decorate your work with watercolour paints or felt pens. You can also use 'test pot' paints from a
hardware shop if you want a bright colour and a shiny surface.
A tip: Cleaning up the mess. It is best to work in the kitchen or outside. Gypsum is not very soluble so be careful how
much goes down the sink - you might block it up! A little runny plaster with lots of water won't hurt though. Surplus
casting plaster (after the mould is full) is best left in the mixing container to set. It will come out very easily then.
A tip: Your waste plaster is really good for your garden so spread it around -
especially in places where drainage is bad! It will disappear after a few weeks (it dissolves slowly in the rain).
More things to do
It is fun to make your own moulds. You can use lots of different materials to make a mould - anything soft. We talk about
mud, playdough, part-set plaster and Lego!
Mud: Clay soils make the best mud. Get your mud from about 10 cm down where there are fewer bits of stick and leaf
that must be picked out! Knead your mud between your fingers adding a little water to make the mud a bit easier to squash
around - make it squashy but not runny. Squeeze your mud into a ball and then squash this into a circle a bit bigger than
your hand and about 2 cm thick.
Now wash the mud off your hands. When they are clean and dry, rub in some Vaseline or cooking oil - not too much, just
enough to make you hands a bit greasy. Next, press your hand into the mud to leave a hand print about 1 cm deep. This is
your new mould! The oil on your skin will help to stop the mud sticking to your hand so that it leaves a nice impression.
A wall: Cut some long strips of card (from a cereal packet) about 2 cm wide. Use a stapler to join these together into
one long strip. Bend this round to make a circle a bit bigger than your handprint and staple the ends together. Gently
press your circle into the mud around your handprint to make a little wall. This will stop the plaster from running all
over the place.
Next, make up some plaster of Paris and pour it (gently) into the mould.
While the plaster is still very soft, half-bury a loop of string at one end.
After about 2 hours remove the plaster of Paris from the mud, peel off the card 'wall' and wash off the remaining mud
from the casting under running water. A soft brush will help. This makes a really nice wall plaque.
A tip: Try bending the card wall into an oval or heart shape - this makes an oval or heart-shaped wall plaque!
An idea: Make a mud impression and cast of your new baby brother's hand or foot. It will make a great record of how tiny
he was. Don't forget to write the date on the casting - and to wash the baby afterwards!
An idea: Make a mud impression and plaster cast of the giant fish you caught! Then wash the fish and cook and eat it!
An idea: Make a cast of a wildlife or pet (or burglar!) footprint that you found in the mud out in the garden.
Other moulds:
Instead of mud, you can use other soft materials to make impression moulds - Playdough works well - for
a Playdough recipe visit HomeSchoolZone.
You can also use plasticine or candle wax but they must be warmed to make them soft enough. Put them in the sun or in
front of a heater. If you are really careful you will even be able to see your fingerprints in these!
Part-set plaster of Paris:
This is also good for making impression moulds. Mix up some plaster of Paris (a slightly more
concentrated mix is best). Pour it into a shallow dish or a large plate and leave it for 20-25 minutes until part
set. Then press your hand into the part-set plaster and keep it really still for 5 minutes - the longer the better.
You should get yourself comfortable before making your handprint then 5-10 minutes will not seem so long. You will
find it hard not to wriggle! If you can wait till the plaster is hard before you remove your hand you should
have a really nice impression - with all your fingerprints showing.
'Waste' moulding: Now, a choice! You can either keep this plaster impression as a wall plaque OR you can use it as a "waste" mould to make
a cast of your hand. To use it as a waste mould, let your plaster impression dry right out. Then, grease it lightly
but all over with cooking oil or Vaseline and pour fresh plaster on top (about 1 cm deep).
Wait at least 4 hours till this new lot of plaster is set hard. Then you can 'waste' the plaster impression mould by
chipping it away from the casting of your handprint using a hammer and chisel. We managed to get our cast out without
wasting the mould! The plaster impression mould is on the left - the casting is on the right.
For bigger kids:
Using plaster of Paris, you can make model buildings from pre-cast parts just like real builders make
bridges and industrial buildings from pre-cast concrete. We suggest that you choose a simple building shape to
start with. First you design your model as a series of flat panels (floor, walls and roof etc), then you build
Lego boxing (shuttering) to make the moulds for these. You should include doorways and windows in the walls!
Lego is really good for making angular shapes for buildings.
It is best to smear the Lego bricks with cooking oil (so that the plaster does not stick) and to lay them
on smooth plastic resting on a flat board. Make up your plaster slightly more concentrated than usual so that it
does not run out below the Lego bricks. Allow the panels to set very thoroughly before removing them from the
mould or you could break them (3-4 h). When all the panels are dry you can glue them together to make the
final structure. Use PVA wood glue or wet plaster as 'cement'. Afterwards, soak the Lego bricks in water and
any plaster will come off easily with a nailbrush.
School projects:
How strong is plaster? Use Lego bricks to make a mould for a pre-cast plaster 'beam'. You can measure
the strength of your beam using thin string to hang a large plastic bottle (exactly) from the middle, while the ends
of the beam are supported on blocks. Slowly fill the bottle with water and stop when the beam breaks. Write down
how much water was in the bottle when the beam broke (either the weight or the volume - remember, 1 litre of water
weighs 1 kg). How strong was your beam?
An idea: Does plaster get stronger as it 'ages'. Cast a number of beams and release them from the mould exactly 2 hours
after pouring. Write the pouring time and date on each beam lightly with a soft pencil and put it in a plastic bag
so that it does not dry out. Cast a new beam every few hours for 2 or 3 days then test all your beams for
strength. Is old plaster stronger than young plaster?
An idea: Do plaster beams get stronger as they dry out? Cast a number of beams and let them dry for different lengths
of time after pouring (say, from 2 hours to 2 days drying). Strength-test your beams. Is dry plaster stronger than
wet plaster?
An idea: Can you re-enforce plaster? Try putting some strong fibres into the plaster immediately after pouring while
it is still runny. You can try anything: hair, wool, cotton wool or grass - the ancient Egyptians used straw - see
Exodus Chapter 5 verse 6 in the bible! Is re-enforced plaster stronger than ordinary plaster?
