Demiglace is actually any rich stock or broth that is reduced by half or three-quarters to enrich it; it is normally still a relatively thin liquid. Glace de viande is made the same way, but it is normally reduced by a factor of 8-10, has a thick syrupy or paste-like consistency, and is much richer. Glace de viande can sometimes be purchased, but is expensive, $30-40 per pint, and if you look at the list of ingredients, often has additives and other adulterants, which make it easier to make. This recipe is "the real thing", is easy to make, though time consuming, and much cheaper than the imitations you can buy. A little goes a long, long way, and it stores indefinitely in the freezer. Just knock off a couple tablespoonfuls, and add to any sauce to flavor it.