This section is from the book "The American Woman's Cook Book", by Ruth Berolzheimer. Also available from Amazon: The Domestic Arts Edition of the American Woman's Cook Book.
The simplest and most accurate method of determining whether the sirup is thick enough for your purpose is to measure its temperature, because the temperature rises steadily as the sirup thickens.
A Candy Thermometer registering up to 3 50° F. is not expensive, and it will not only give you a higher average of success in candy making but will save you the time and labor that must otherwise be given to testing the sirup. A table giving the various stages of sugar cookery will be found on page 12.
If You Are Not Provided With a Thermometer, the following test will help you to determine when to take your candy from the fire.
Soft ball stage (for fondant and fudge) the sirup forms a soft ball which loses its shape immediately when removed from the water.
Light to medium crack stage (for toffee and butterscotch and hard candies to be pulled) the sirup forms spirals or threads which are brittle under water but which soften when removed from the water and stick to the teeth when chewed.
Hard crack stage (for clear brittle candies) the sirup forms spirals or threads which are brittle when removed from the water and do not stick to the teeth when chewed.
Creamy Candies - Creaminess is desirable in soft candies. "Creamy" means that the texture should be very smooth, not grainy at all; soft but not sticky. This means that the sugar must not remain as a sirup, but must crystallize. The crystals, however, must be very fine, so that they can not be felt by the fingers or in the mouth.
Creamy candy should not be overcooked. If it reaches too high a temperature, accidentally, a little water may be added and it may be recooked to the correct temperature. This does not give as good a result as one cooking to the correct temperature, but it improves a poor product.
Creamy candy should be cooled before it is beaten. Beating candy while it is hot causes large crystals to form and grainy candy results. If crystals that form on the side of the pan in which candy is cooked fall back into the candy, they tend to cause large crystals to form and to make grainy candy.
A small amount of corn sirup tends to prevent grainy candy. Creamy candies made with corn sirup will require longer beating before crystallization takes place than will candies made from all granulated sugar. They also soften more quickly on standing. If too much sirup is. used, the candy will not crystallize at all and the best thing to do with it is to boil it until it reaches the proper stage for a pulled or brittle candy.
One-eighth teaspoon of cream of tartar or one-half teaspoon of lemon-juice or acetic acid to two cups of sugar may be used instead of corn sirup or glucose. They change part of the granulated sugar to glucose during the cooking process.