Agar, also called agar agar, is a carbohydrate-based gelling product derived from seaweed. It can be used in many of the same ways as gelatin, and since most commercial gelatin contains pork, agar is popular with vegetarians and those who don't eat pork for religious reasons. Agar is more powerful than gelatin, so less is needed to set a given quantity of liquid. Unless you buy your agar premeasured in individual envelopes, you will need a high-quality kitchen scale able to measure single grams accurately.
Open the envelope of agar powder or flakes, and empty it into a saucepan. Alternatively, if your agar came in the form of a bar, grate it or chop it very finely. Weigh out 2 grams of grated agar, or bulk flake or powder agar, using a very accurate kitchen scale. Pour this into the saucepan.
Add a neutral or flavored cooking liquid, such as fruit juice to the saucepan. Allow the agar to soak in the cold liquid for five minutes, then turn the burner to medium-high heat.
Make a note of the time when the agar solution comes to a boil. It must boil for a full five minutes in order to set properly. Stir while its cooking to ensure that the agar has completely dissolved, especially if you have grated or chopped the agar from a bar.
Test the quality of the gel by spooning a small amount into a chilled bowl. It should set to the desired consistency within 20 or 30 seconds. If it is too stiff, add more of the cooking liquid. If it is too soft, add a few more crumbs of agar.
Pour the agar into a bowl or jelly mold to set once you are satisfied that you have the right consistency. Agar will set at room temperature, but it is perishable and should still be refrigerated. Serve warm or cold, as desired.
If you find the agar gel does not have the correct consistency, you can melt it and correct the texture by adding more agar or more liquid, as necessary. Two grams of agar will set 2 cups of water to a soft gel, but more would be needed for an acidic liquid such as fruit juice.
Agar will not melt once it has gelled until it reaches a temperature of 185 degrees F. That makes it possible to make gels that will hold their shape and texture in hot soup or other dishes, creating interesting possibilities for the cook. Unfortunately it also means that agar-based gels do not melt in the mouth like regular gelatin, which some diners will find disconcerting.