The primary difference between a glaze and a demi-glace is that demi-glace is a half glaze, which means that demi-glace is a rich brown sauce that is made of equal parts of beef or veal stock and Espagnole sauce, which is a brown sauce and reduced by half.
Glace or demi-glace is a much more concentrated, syrupy reduction of stock that is made by reducing a stock by a much larger factor, and often to one tenth of it's original volume.
Demi-glace is also more liquid and contains ingredients such a stomato and flour from the Espagnole, and glace is a pure intensely flavored syrup from the stock itself.
Demi-glace is a child sauce of the Espagnole sauce, which is another mother sauce.
Demi-glace is basically a highly reduced and concentrated brown sauce that is made from equal parts of Espagnole sauce and brown stock and is used as a rich base for many other classic French brown sauces, stews and soups.
The mother sauce that is typically used as a base for making a demi-glace is Espagnole sauce or brown sauce.
Demi-glace is a reduce of equal parts of Espagnole sauce and brown stock, that is simmered until it becomes a thick and intensely flavorful sauce,
Demi glace is often served with meats like duck meat, lamb meat or steak.
Espagnole sauce is a rich and dark brown sauce that is made from veal stock or brown beef, a dark brown roux of butter and flour cooked until very dark and mirepoix, "diced carrots, celery and onions", and tomato puree.
Espagnole sauce provides a deep and rich flavor base for other sauces.
And it's a foundation for demi-glace and several other classic French sauces, like Chasseur and Bordelaise.
To make Demi-Glace you mix Espagnole sauce with equal parts of brown beef or veal stock.
Then simmer the mixture uncovered, allowing it to reduce and also intensify in flavor until it is a thick and velvety consistency.
Once done the final product of demi-glace is a highly concentrated and flavorful sauce.
Demi-glace sauce is a basically a highly reduced brown stock, that is traditionally made from roasted beef or veal bones, mirepoix (onions, carrots, and celery), tomato paste, flour for the roux, and aromatics like garlic, bay leaves, thyme, and peppercorns.
Demi-glace sauce can also be made by combining a premade brown stock (like Espagnole sauce), which contains mirepoix, tomato, and roux, with more beef or veal stock and wine.