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Preparation with potassium hydroxide

KOH or Potassium hydroxide is a strong base that can be used for the preparation of some calcareous fossils (mergelige kalksteen). It is often used for the preparation of fossil echinoids. KOH is available in a solid flake form from your local pharmacy. It is strongly hygroscopic, so it needs to be kept dry and sealed well. The chemical reaction is initiated by adding water to the flakes.


Attention! KOH is highly corrosive to skin and eyes and irritates the respiratory system. Use only in well ventilated areas and wear gloves, goggles and protective clothing. It is necessary to use a glass recipient (no plastic or metal), as KOH creates a heat-releasing (exothermic) reaction with water.


When used correctly KOH can remove almost all remaining matrixwhile preserving all details. However, if this technique is not used in the correct way, it can damage the fossil. For example small cracks or other damage may appear, or the fossil might be bleached. A classic mistake is to give KOH too much time to work. Try the treatment on a testing fossil first, and work in shorter steps that can be repeated if necessary. In case of using more cycles, it is best to cover up the previously treated parts with a glue that is soluble in acetone. This glue can be removed afterwards with acetone or, in some cases, warm water.

In principle, NaOH (Sodium Hydroxide or caustic soda) should work too, but KOH gives better results. NaOH is sold as drain cleaner in a solid form.

A treatment cycle consists of the following phases, which in this example are used on a number of echinoids:

Step 1.

Remove as much of the matrix attached to the echinoid as possible, before starting the treatment, to prevent having to use more cycles of treatment by KOH than strictly necessary.

 (© Marc de Vries)


Step 2.

Wet the echinoid by using a plant sprayer, to make it easier to apply the KOH. Apply the flakes to the entire surface of the echinoid that needs to be cleansed.

(© Marc de Vries)

Step 3.

Moisten the KOH with a plant sprayer or pipette to initiate the chemical reaction. Use water sparingly to prevent rinsing off the KOH. A humidity of 80 percent or more often is enough to spontaneously start the reaction.

 (© Marc de Vries)

Step 4.

Wait for 2 or 3 hours (or shorter, depending on the fossil) for the KOH to work. Repeatedly check to make sure the fossil doesn't corrode.

(© Marc de Vries)

Step 5.

Clean the fossil in clean water with a hard brush (no copper, messing or iron).

(© Marc de Vries)

Step 6.

Put the fossil in vinegar for a 5 minute maximum, to neutralise the remaining KOH.

 (© Marc de Vries)

Step 7.

Brush the fossil again and place it in clean water. Refresh the water for about 5 times and leave it under water for an entire day.

(© Marc de Vries)

Step 8.

The process has been completed now. It is recommended to wrap the fossil in a tissue to drain the last residue of vinegar and KOH from the fossil.


Many thanks to Marc de Vries (Texas Fossils); translation FL

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