Most casts applied in orthopedic offices today in the US are fiberglass casts. Plaster used to be the only available material suitable for casts.
Plaster strips or rolls are made by coating gauze with Plaster of Paris. A plaster cast is made by wetting these strips and rolling them onto the patient’s arm, usually over some cast padding material.
Plaster of Paris is calcined gypsum, which is a fine powder.
Adding water to this powder, then letting it dry (when a cast is put on), creates calcium sulfate, a hard white substance, like chalk.
Hardening of a plaster cast usually takes 5-10 minutes and heat is created during the hardening process. Most children say the cast feels very warm but if the arm has been padded well, the sensation of heat is not as intense.
Fiberglass is a type of plastic that is easily shaped. Most fiberglass casts are made from rolls of material that is soft inside the package, and impregnated with polyurethane resin.
Water activates the resin and creates a hard cast in about 3-4 minutes.
Fiberglass casts are applied in a similar way to plaster casts. The roll is wrapped over padding around the arm and creates some heat during the hardening process.
Why your doctor might use plaster versus fiberglass
In general, fiberglass casts are lighter and can get wet if the right kind of padding is used. Therefore these are usually better for kids who have broken arms in the summertime. They’re cleaner to put on in the doctor’s office, and you can see the bones better through a fiberglass cast than you can through a plaster one.
So why do some specialists use plaster?
Plaster casts can be molded and shaped easier than fiberglass. If your doctor is trying to “mold” or put some gentle pressure on the arm around the fracture, he may use a plaster cast.
I find that plaster sets up and hardens faster than fiberglass, so if I set a bone in the office or in the emergency room, I can hold the cast still for a few minutes and let go when it’s hard, knowing that the bones won’t move. Fiberglass casts and splints don’t harden that rapidly.
Fiberglass cast material will stick firmly to exposed skin and clothing when wet, so don’t touch the cast material while it’s being put on; try to keep hair and clothes away from it while the doctor or nurse is putting the cast on.
You can read more about the composition and technical details of fiberglass casting material in this PDF file from the 3M company on Scotchcast Casting Products.