So, you want to make an authentic Rock and Rye, or a Rock and Rum, or a Rock and Rack? Maybe you want to make some Tiki drink with a more traditional version of a certain syrup than was available when the drink was first created in the middle of the 20th century. Either way, you will need genuine rock candy syrup.
The above is from the American Journal of Pharmacy, volume 67, in 1895. It indicates that rock candy syrup is the left-over syrup from the production of rock candy. As such, no sugar should be left to crystallize out of the syrup. In New York State, in 1873, a case was brought to court over an alleged deficiency found in barrels of rock candy syrup. The plaintiffs made clear that they had desired high quality rock candy syrup “that would not crystallize, or the sugar fall down.”
Around the same time, admonitions can be found advising to obtain “rock candy syrup from a rock candy manufacturer.”
Below is from the Western Druggist, Volume 17 in 1895:
Volume eleven of the Bulletin of Pharmacy in 1897, specifies that when cooking the syrup from which rock candy will be produced, no temperature higher than 112º Fahrenheit should be allowed.
The 112º Fahrenheit limit is kept to prevent the conversion of sucrose into invert glucose. Some amount of conversion from sucrose into glucose may be considered acceptable by some people making simple syrup, but it isn’t okay for the production of rock candy. If you want to produce rock candy, you’d best stick to low temperatures and pure sucrose and pure water — in super-saturation so that rock candy will form. Once all of the excess sugar has crystallized out, you will have rock candy syrup.
So, rock candy syrup and buttermilk have this in common: they were both traditionally the stuff left-over after the making of the named thing (rock candy or butter), but are now routinely divorced from those processes and are simply compounded independently.
The above ingredients for Amoretti brand so-called ‘rock candy syrup’ show that, though it may be a heavy syrup, it is definitely not traditional rock candy syrup. Traditional rock candy syrup would not contain dextrose or fructose. Genuine rock candy syrup carefully contains only sucrose and water. Why it would need dextrose, fructose and “natural flavor” is a mystery to me!
So, what if you want to have traditional rock candy syrup? Some old sources state that one of the reasons for the production of so much imitation rock candy syrup is that making the real thing via rock candy production takes time. Unfortunately, that is true. But, if you don’t care how much rock candy you make, the time needed can be drastically shortened.
Bring one part (by volume) distilled water to a temperature as close as possible to, but not higher than, 112º Fahrenheit. An induction cook-top (like this one) that can be set at exactly 110º Fahrenheit could be really useful. Add three parts (by volume) pure, refined sugar (sucrose). Stir over the same heat (never exceeding it!) until the sugar is dissolved. It may take some time. Pour the syrup into mason jars. If you want to harvest the rock candy instead of discarding it (stuck to the inside of the jar), suspend a string in the mason jar to which the rock candy will form. Once no more rock candy forms inside the syrup, pour it from the mason jar into a syrup bottle. This syrup will contain the highest possible amount of sugar in a stable solution. It will be the closest thing to genuine and traditional rock candy syrup that you will have ever had.