How to Dissolve Limestone off of Silica Fossils

Fossils such as this ammonite are easier to identify when removed from the matrix rock.

Well-exposed fossil specimens are easier to identify and typically sell for higher prices. Since most fossils have the same mineralogy as the matrix, or rock in which they are contained, exposing a fossil can require painstaking work and a good deal of luck. In the case of fossils that have a different mineralogy, however, removing the matrix may be simpler. Fossils that are composed of silica (silicon dioxide, or quartz) found in a matrix of limestone are one such case.

Things You'll Need

• Diluted or weak acid
• Clean water
• Plastic bucket or tub
• Rubber gloves
• Soft-bristled brush
• Baking powder

Instructions

1. Reduce the size of the specimen by cutting or breaking off excess matrix. Be very careful not to break through to the fossil specimen.

2. Prepare a bath of a weak acid such as acetic acid (vinegar), used straight and poured into a plastic bucket or tub. Muriatic acid, available at hardware and pool-supply stores, may be used instead of acetic acid; dilute this stronger acid heavily. To dilute, fill a plastic bucket or tub with one to two gallons of fresh water. Pour a small amount of muriatic acid---less than a cup---into the water. Do not allow the acid to splash on your skin or clothes.

3. Immerse the fossil specimen in the acid solution. The limestone will begin to bubble and fizz as it reacts with the acid. Use a pair of rubber dishwashing gloves to remove the specimens from the acid bath occasionally. Rinse the specimens with fresh water to check progress. When the limestone matrix has been dissolved, clean off any remaining debris with a soft-bristle brush and rinse the specimens thoroughly. Set the fossils in a safe place to dry.

4. Neutralize the remaining acid by adding baking soda until the mixture no longer fizzes. At that point, the solution is safe to pour down the drain. Rinse the sink and bucket well and flush completely with fresh water.

Tips & Warnings

• Test the effects of the acid by dripping a small amount on the fossil or a fragment of the same composition before immersing the full specimen in an acid bath.

• Always wear eye and skin protection when handling acids.

References

• "Fossil Collecting in the Mid-Atlantic States"; Jasper Burns; 1991
• "Introduction to Fossil Collecting"; Glen Kuban; 2004