If any of your friends has had an accident, you’ll see the hurt arm covered in plaster. Why do doctors do this? Chemistry tells us why.
Why do casts work?
The best reason is that they are so hard, that the bone cannot go out of alignment again. Also, they can be made to suit the shape of your arm, so they fit really snug and are not uncomfortable. And they are fairly cheap and quick to make.
There are some problems too. They are not waterproof, so you have some trouble while bathing or washing hands. If water got in, it would erode the cast. Also, they can be quite heavy. And as your wound heals, your arm needs more flexibility. Then you have to break the old cast and make it again.
How a cast is made
When you have a fracture, the doctor will first clean up the wounds, reset the bone and apply medicine. He’ll then wrap some bandages to protect the wound. Now watch the doctor’s attendant carefully. He’ll take some packets of gypsum and mix them with water. He’ll dip bandages in them and wrap them around the fracture.
Does it feel warm? That’s because the gypsum in the plaster (which is really calcium sulfate) reacts with water to make an insoluble form. This is a sticky form that settles like cement. Over time, the plaster will become hard and dry, and make a firm case for your fracture. Now your body can take over the healing process.
Sometimes you may find that there is no alternative for true plaster of Paris. Because plaster is gypsum-based, it has applications that cannot be achieved by glue or flour methods. Fortunately, plaster of Paris mix is relatively cheap and just as easy to mix as any of its alternatives. Combine the mix with an appropriate amount of water, as specified by the packaging. Use a stick or heavy spoon to stir until well mixed; you will have a thick paste similar to the glue mixture but heavier. This plaster mix can then be poured into molds or sculpted over a rigid structure to create masks, figurines and so forth. Once molded or poured, it takes about an hour to dry into a solid, but it’s best to leave it for an entire day to dry completely through.
For added color to your projects you can stir a small amount of paint into the plaster mix. One colorful plaster project that’s fun for kids simply cannot be replicated with plaster alternatives, because it requires the chalky qualities of actual plaster of Paris. Add color to the plaster mix and pour it into molds made of toilet paper tubes. Once the plaster hardens, cut away the tube and the resulting product can be used as sidewalk chalk!
Nowadays doctors suggest you use a fibreglass cast instead. It’s much lighter, waterproof and flexible. But it is still expensive, and you cannot autograph it!