From: The respiratory exchange of animals and man by August Krogh, 1916, page 40.

“Masks of rubber which can be fitted on the face and enclose the mouth and nose are far more convenient than mouthpieces, but it is extremely difficult to avoid leakage.  Bohr constructed masks which were specially fitted to each person on whom it was intended to experiment.  These masks (fig. 13) consist of a funnel-shaped piece of tinplate coated on the edge with a substance used by dentists and know commercially as Stent’s composition.  This substance becomes soft at a temperature of about 50 degrees, and can then easily be moulded on the face of a person and can be made to fit absolutely airtight when greased with lanolin, causing at the same time a minimum of inconvenience. These masks are much used in Danish laboratories for all experiments which have to last more than a few minutes at a time.”