Despite it begin the current dietary bad guy you just can’t deny what an amazing substance sugar is. Even if you forget about the taste we all love, purely as a creative material in cooking it’s versatility is incredible. You can even use sugar to learn about the science of crystals. What more could you ask for than running a science project that in the end you can eat the results. I also think rock candy sticks are wonderfully pretty to look at and photograph and I had a lot of fun with that. Growing rock candy may test the patience of younger children but if there’s just a little bit of OCD in any of you parents, like me, you will love this process.
The rock candy recipe
- 1 part water
- 3 parts sugar
- Wooden skewers
- Tall thin glasses or jars
- Food coloring and flavouring if desired.
- Clothes pegs or large bull dog clips for suspending the skewers over the glasses/jars
The ingredients couldn’t be simpler but as always working with sugar can be both tricky and dangerous when it gets hot, so the first stage definitely requires adult supervision. All recipes call for one part water to two to three parts sugar. So if you had one cup of water you’d need two to three cups of sugar. Basically the more sugar you can dissolve into the water the better – though there is a catch – if you add too much sugar and do too much stirring your sugar syrup may crystallise in the pan. This is want you don’t want and the major omission that I found in all the tutorials I read. Obviously the more sugar and water you use the more rock candy you can make but I’d suggest starting with 1 cup water: 3 cups sugar for your first batch until you have the method working so as not to waste too much sugar.
Method
Wet your skewers and roll them in sugar and hang them to dry in the jars. The sugar on the skewers provides a ‘seed’ for the crystal growth. Do this as far in advance as you can. If you get nice and evenly coated skewers you should get nice even crystal growth, but they need time to dry so they don’t wash straight off when you put the skewers into the solution.
Add one cup of water and three cups of sugar to a heavy based pan.
Bring to the boil over a medium high heat stirring UNTIL the syrup boils. As soon as it boils STOP STIRRING. Bring to a rolling boil and then take off the heat. This should only take a couple of minutes. The rest of the sugar should now be dissolved. You should end up with a clear, thick liquid.
We put different colouring in each glass so we ended up with 5 different colours. It’s up to you if you want to make them all the same. If you want a different colour for each you can put a few drops of colouring (concentrated gel is best) in the bottom of each glass
Allow your syrup to cool for 15 to 20 minutes and warm your glasses to avoid the hot liquid cracking them. Carefully pour in your hot syrup to the top mixing the colouring as you go.
Lower your sugar seeded skewers into the liquid. Depending on the size of your glass/jar you will be able to fit either one or two in each. Make sure the skewers don’t touch the bottom or sides of the container.
Put the containers in a warm sunny places where they won’t be disturbed and loosely cover with a kitchen towel if you have dust or bugs to worry about – and wait!
Within a day or two you should start to see some growth. If you start to find a sugar skin start to form on the surface of the liquid or crystals form at the bottom of the container it’s a good idea to pour the liquid into a fresh container. Use a sieve to remove and chunks of sugar floating around. Do this once a day at the most.
It should take about a week to grow some decent sized crystals.
The science
So this is all great fun and produces some yummy results but we’re also here to learn something right! Now I only studied chemistry as far as high school so I’m no expert but again thank goodness for all the helpful people out there in cyberspace all the answers are there.
So what’s happening is when we melt the sugar in the water we are creating a SUPER SATURATED liquid which is essentially unstable and the sugar can not stay dissolved in the solution once it cools. As the liquid cools the sugar comes out of the solution it is forming a precipitate, in the form of sugar crystals and the method is called precipitation. The other effect that makes the crystals grow is evaporation – over time the water evaporates from the solution again increasing the saturation of sugar in the solution and encourages the sugar molecules to collect on your stick.
Molecular bonds are soft with relatively low melting points. Rock candy sugar crystals are made from molecular bonds (there are other types of bonds in crystals such as ionic in salt) and they grow molecule by molecule on the stick. There may be as many as a quadrillion molecules attached to your stick when they are fully grown.
Links
Here are some of the useful websites that helped us with our rock candy plus some science sites for further reading.