Casts are the main method that doctors use to immobilise a broken arm. In the past, casts were made of plaster, while fibreglass is typically used today. If you need a fake cast for a costume or play, you can make one fairly easily out of paper mache. This cast will be removable, and can be custom made for a specific person. When you pair it with a sling, you can make it look like your arm is really broken.
Tear newspaper into strips that are 6 inches long and 2 inches wide.
Mix a paste that is one part thick white glue to one part water.
Have a model hold his arm with the elbow bent at a 90-degree angle. If you have access to a mannequin or other fake arm with the arm bent, you can use it instead of a live model.
Wrap three layers of gauze loosely around the model's arm, going from mid-bicep to the wrist. Wrap the webbing between the thumb and first finger as pictured.
Dip the first newspaper strip into the paste, then squeeze off the excess and smooth it onto the gauze.
Continue adding overlapping pieces of newspaper until the entire gauze area is covered. Leave half an inch of gauze uncovered at both ends.
Apply three additional layers of newspaper strips, each dipped in the paste.
Allow the cast to dry. You can speed this process with a hairdryer, but be careful not to burn the model.
Cut the cast along the back side with a sharp pair of scissors to remove it from the model's arm. This should be fairly easy if the gauze was wrapped loosely.
Paint the cast white or dark blue. If desired, use coloured markers to write messages from friends and well-wishers.
Hide the cut back of the cast against your body when wearing it.
Stop wrapping the gauze just below the elbow for a forearm cast.