In the culinary arts, the word glace refers to a thick, syrup-like reduction of stock which is in turn used to flavor other sauces. The word glace means "glaze" or "ice" in French and it is pronounced "GLOSS."
A demi-glace is a rich, dark sauce made by combining half brown stock and half brown sauce (called Espagnole sauce) and then reducing that by half (demi means "half").
Glaces differ from demi-glace in that they are much more concentrated. For example, glace de viande is stock that is reduced by a factor of eight to 10 until it is a syrup, while in a demi-glace the stock is reduced by only a factor of two to four. For this reason, if you substitute glace for demi-glace you should use only half as much.
Making and Using Glaces
Glaces are convenient because just a small spoonful can add a lot of flavor to a sauce or soup. Glaces freeze easily, and it's easy to turn a glace back into stock again simply by adding water.
A typical glace recipe starts with an unsalted stock of some kind. Glace de viande, or meat glace, is made from brown stock. Chicken glace, or glace de volaille, is made from chicken stock. Fish glace, or glace de poisson, is made from fish stock.
If you start from scratch, you would first need to make the stock. In a big commercial kitchen, this is often a daily process as bones are simmered with vegetables to make the stock. At home, you might skip that process and buy already prepared stock. Then bring the stock to a boil and simmer, skimming any solids if they appear. When it has been reduced by half, the stock is strained. The stock is then returned to simmering until it is reduced to the point where it is syrupy.
One word of caution: When making your own glaces, be sure to use unsalted stocks, as the reduction process is such that any amount of salt in the stock will be intensely concentrated in the final glace, making it way too salty.
Easy Demi-Glace Recipe
How to Use Demi-Glace
A basic demi-glace recipe uses both brown stock and Espagnole sauce, which includes tomato puree. As with glace, you could start from square one to make the stock and the sauce from bones and mirepoix (chopped onions, celery, and carrots). An easy demi-glace recipe for home cooks starts with store-bought beef stock.
You can use demi-glace as the base for many other sauces. A classic red wine sauce can be made by adding some red wine and reducing it a bit. You can make a traditional mushroom sauce by adding mushrooms and shallots. These sauces can be used to finish a roasted meat dish such as a steak or pork chop. Demi-glace is added to soups and stews to deepen the flavor and to braising liquids for large cuts of meat such as lamb shanks. A dollop in a stir-fry will also boost the flavor.