Limestone fossils are among the most interesting rocks to collect. These mineralised animal and plant remains represent a moment in time, captured in stone. Finding them is only half the adventure. Preparing the fossil by releasing it from the matrix of surrounding rock and preserving it properly will yield a geological treasure that will last a lifetime and beyond.
- Skill level:
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Things you need
- Hobby knife
- Hand-held power grinding tool
- Fine chisel
- Large sewing needle or dental pick
- Pin vice (if using a sewing needle)
- Microscope (optional)
- Consolidant (optional)
- Resin (such as Vinac B-15 or Butvar B-76)
- Glass jar
Remove any sand, silt or other loose material around the limestone fossil with a soft brush and a hobby knife.
Use a power grinding tool, such as a Dremel tool, to remove large amounts of matrix rock from around the fossil to make the specimen easier to handle and work with. Use a sharp carbide bit to avoid removing large chunks of rock and damaging the fossil.
Remove the matrix rock closer to the fossil with a fine hand chisel and mallet. Work up to the rock within a few millimetres of the fossil.
Use a pointed tool, such as a large sewing needle in a pin vice or a dental pick, to work at the last bits of matrix left on the fossil. Be careful not to damage the fossil by exerting too much pressure. Use a microscope at 5 to 10 power to do extra fine work, if necessary.
If desired, apply a consolidant to protect the limestone fossil once it is free from the matrix. While it is usually not necessary, using a consolidant, a type of hardener that penetrates the rock, can help to preserve the fossil. Allow the fossil to dry completely before proceeding.
In the glass jar, dissolve a resin such as Vinac B-15 or Butvar B-76 in acetone at 1 part resin to 32 parts acetone.
Brush the mixture onto the fossil, allowing it to penetrate the rock. Allow the resin to dry thoroughly.
Tips and warnings
- Take your time when working with limestone fossils. The process of freeing a fossil from the matrix rock is laborious and time consuming. Be prepared to spend days or even weeks working on the fossil.
- An electric engraving tool, air abrasive tools that act as tiny sand blasters, and airscribes that drive miniature chiselling tools may also be used to do fine work on fossils.
- Do not use acids to dissolve matrix rock when trying to free or clean a limestone fossil. Limestone fossils are usually made of calcium-based rock, such as calcite. Acids react with calcium and may irreparably damage the fossil.
- Do not use polyurethane or other surface treatment as a consolidant.
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