I'm trying to make a relief sculpture copy out of sugar glass. I don't live in the US, so the corn syrup is not easily available. I read in multiple recipes that cream of tartar can be substituted with any edible acid, like citric acid.
I'd like to know more about the chemistry behind the ingredients, and find edible substitutes. Additionally, how can I make sure that it doesn't lose transparency and go brown? And what about preventing it from cracking too soon?
I was composing as @Melissa was responding... here's some more detail to append to my comment above.
Sucrose is a disaccharide consisting of fructose and glucose. When boiled with water, it forms a syrup. When this syrup cools, depending on the relative concentrations of sugar and water, different properties will result (e.g., liquid, thread, firmball, soft crack, ...). However, sucrose by itself has impurities; these impurities will tend to cause (or help to form) small grainy crystals (think granulated sugar) instead of a smooth, transparent, uniform, glass-like candy. This potential for crystal formation is why, e.g, ...
As noted by @Melissa, other sugars (such as fructose and glucose are found in corn syrup), are added that help to reduce the likelihood of "bad" (undesired) crystal formation. You should be able to find corn syrup or other alternatives by mail order. Other options include pure glucose syrup.
You may be able to find golden syrup, which has basically similar properties to corn syrup, but it probably has too many other impurities to make it suitable for candy making. I don't know about maple syrup, honey, or agave syrup.
Your question about acid is an excellent one: Acids aid in the formation of invert sugar. Invert sugar is sucrose ("table sugar") that has been (partially or completely) chemically broken down into its constituent monosaccharides: glucose and fructose. Heating alone will start this "inversion" of sucrose into glucose and fructose; acids will help speed this reaction.
Other possible acids include citric acid, but I wouldn't recommend lemon juice; again, it's got too many other impurities. Powdered, purified citric acid can often be found at Indian grocers; it's useful for cheese-making also.
Basically, the addition of corn syrup and the addition of acid fill a similar purpose: providing monosaccharides to prevent the formation of undesirable sucrose crystals in your cooling sugar syrup. Strictly speaking, you don't need to add anything to get a glass-like candy; however, it's difficult. Adding invert sugar or catalyzing invert sugar formation with an acid will help improve the margin for error.
Also, as I said in comment above, you might consider using isomalt instead of sugar (i.e., only isomalt in place of all the other sugar; no sucrose, no corn syrup, no acid). It's basically edible in small quantities, but I wouldn't eat a lot of it (of course, I wouldn't eat much sugar, either...:). Please look up information on the edibility and safety for more information on the quantities you're discussing. That said, sugar sculptures aren't often intended for actual consumption...? Isomalt melts directly (no water needed) and doesn't crystallize as quickly as sucrose. It's more expensive but easier to work with in some ways. It's less hygroscopic (water absorption) than sucrose, and won't caramelize (browning) as sucrose.
HTH; respond to comments and I could provide more detail if helpful. Good luck with your sugarwork!
When you make sugar glass, the role of the corn syrup is to halt crystallization.
This is needed because otherwise the sucrose molecules will form around any impurity in your sugar mixture.
If you do not have access to corn syrup I would suggest finding other ways of halting the crystallization process such as: lemon juice, maple syrup,honey and agave syrup.
Good luck!
