This section is from the book "The Epicurean", by Charles Ranhofer. Also available from Amazon: The Epicurean, a Complete Treatise of Analytical and Practical Studies on the Culinary Art.
In former days sugars were less refined than in our time, therefore it was most important that they should undergo the operation of clarification, or, as the very word implies rid them of their impurities and make them perfectly clear. To-day this operation is almost useless, however, in case of necessity we will give the exact manner of proceeding, for it may sometimes be found useful. Put twenty pounds of sugar into a copper basin, melt it with two-thirds of its quantity of water, or one-half pint of water to each pound of sugar, set it on the fire, and when the scum begins to rise, throw in some egg-whites beaten up with water, the proportions being one white for each quart of water; do not stir it again, but let it rise to the surface twice, then pour in half a pint of clear water without eggs; let it rise a third time, and as it does so, remove from off the fire and skim it. Return it to one side of the fire to let it boil and drive the scum on one side of the basin, skim this off as quickly as it gathers. Soon the sugar will become very fine, clear and transparent, but if otherwise, then let it boil till it clarifies thoroughly and pass it through the flannel bag. Sugar clarified by this process is ready to be submitted to all kinds of cooking which we explain further on.
The cooking of sugar is easily measured by a thermometer, but a clever workman will quickly find it out by the mere touch. These various cookings take different names which we will now endeavor to explain.
Cook the sugar, and in order to be sure that it has reached the first cooking, take out a little of the sugar with a spoon, dip the index finger in it and apply the finger to the thumb; separate the two lingers immediately, the sugar should then form a small thread, the thermometer marking two hundred and fifteen degrees Fahrenheit, (one hundred and one degrees Centigrade).
At two hundred and seventeen degrees, the sugar stretches a little more between the fingers, it is now cooked to large thread (one hundred and two degrees Centigrade).
The sugar reaches this cooking when between the two fingers it stretches and forms a thread that breaks. The thermometer is then two hundred and twenty degrees (one hundred and five degrees Centigrade).
As soon as the sugar extends from one finger to the other without breaking it has reached large pearl, two hundred and twenty-two degrees Fahrenheit (one hundred and six Centigrade).
Dip a skimmer into the sugar, knock it at once against the edges of the basin, blow through the skimmer so as to make the small bubbles fly out, and when they do so properly, the sugar has reached its degree of cooking. The thermometer now marks two hundred and thirty degrees Fahrenheit (one hundred and ten degrees Centigrade).
Dip the finger first into cold water, then in the sugar, and immediately into water; if the sugar has reached to proper cooking or small ball, it can be rolled into a soft ball between the fingers, two hundred and thirty-six to two hundred and thirty-eight degrees Fahrenheit (one hundred and fourteen to one hundred and fifteen degrees Centigrade).
When the thermometer reaches two hundred and forty-six to two hundred and forty-eight degrees Fahrenheit or one hundred and nineteen to one hundred and twenty degrees Centigrade, then the ball instead of remaining soft when rolled between the fingers, becomes solid and hard, the sugar has now reached large ball.
Dip the tip of the finger into cold water, then into the sugar and rapidly into cold water again, so as to detach it from the finger; if it has reached its proper cooking it should break. The thermometer is now two hundred and ninety degrees Fahrenheit (one hundred and forty-three degrees Centigrade).
Letting the sugar boil a few minutes longer, it will reach the crack; now dip the finger into cold water, then into the sugar, and again into the water, the sugar must break between the teeth without adhering to them. It reaches this degree when the thermometer is at three hundred and ten degrees Fahrenheit (or one hundred and fifty-four degrees Centigrade).
Tenth. Grand Casse" or Large Crack or Caramel. - This last cooking is exceedingly delicate and requires the most particular care, so as to avoid having the sugar turn black which it is apt to do very easily. When reaching this last cooking, the sugar slightly loses its whiteness and assumes a shade scarcely perceptible to the eye; this is when the thermometer reaches three hundred degrees Farenheit, and it is now time to add to each twenty pounds of sugar, a teaspoonful of lemon juice. Continue the cooking and when the thermometer reaches three hundred and forty-five to three hundred and fifty degrees Fahrenheit, then remove the basin quickly from the fire, and instantly pour its contents on a marble to get cold.
These are the various degrees the cooking of sugar undergoes, practice alone makes perfect in this particular work, which can only be acquired after much study and attention.