If it is wished to preserve the collars, allow them to stand for some time in dilute hydrochloric acid, which will dissolve out the plaster-of-Paris. If the collars are not required, place them in strong nitric acid, which will dissolve the brass. Another method is to make tile marks just above the collars, heat a piece of glass rod or thick iron wire in the blowpipe flame, and place it on the file marks. Often a crack will go right round at once; if not, the crack can usually be obtained after two or three heatings in this way.