The UK's Nationwide Geology Club for Children

Igneous Obsidian Toffee

By Fureya Nelson Riggott

FUREYA NELSON RIGGOTT made a fantastic geological recipe book in a previous Rockstar competition. Here we share three of her recipes to illustrate three different kinds of rock: one sedimentary, one igneous and one metamorphic.

This is the second, or more specifically, igneous obsidian toffee.

Obsidian is a igneous rock type that is a natural glass very rich in silica (around 70%) and low in water. It is formed by the rapid cooling of viscous lava from volcanoes.

Obsidian breaks into very sharp pieces, like window glass does. Window glass has a higher silica content. Glass (natural or man-made) doesn’t contain any crystals, so its jagged edges can be just a few molecules thick and be sharper even than a steel razor blade.

Also check out Fureya’s other delicious recipes:

Virtual Festival of Geology 2020

Featured as part of the Virtual Festival of Geology 2020

Igneous Obsidian Rock

Igneous Obsidian Rock

Igneous Obsidian Toffee

Igneous Obsidian Toffee

Always make sure you have a responsible adult with you to help you prepare and make your toffee

You will need:

500 gm granulated sugar
150 ml water
¼ tsp cream of tartar
75 gm butter
100 gm treacle
100 gm golden syrup

** Make sure you are not allergic to any of the ingredients!

  1. Oil a 19 cm shallow square tin, or line it with Teflon baking sheet.
  2. Measure the sugar and water into a large heavy pan and heat gently until the sugar has dissolved.
  3. Add the rest of the ingredients and bring to the boil.
  4. Using a clean pastry brush, brush the inside of the pan with cold water, just above the level of the mixture.
  5. Bring to the boil. Boil rapidly for 10-15 minutes, or until the temperature reaches the soft crack stage at 143 degrees C.
  6. Pour into the tin.
  7. Cool for 5 minutes. If you want to mark it into squares, do this now with an oiled knife. Leave it to set in a cool place BUT NOT in the fridge. Or, you can leave it to cool and set, and then do the geologist bit and hit it with a (clean) hammer to break it up, and see the sharp edges.
Top tip – this makes a delicious gift for someone you care about!

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Author: Helen Connolly

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