KOH, otherwise known as caustic potash, or potassium hydroxide can be used on a variety of rock types where the fossil is of calcite composition. KOH comes in a flake or a scale form, which are small and solid. It is ineffective on pure limestone, but can be used on marls, marly limestones, oolites, clays and more. It can preserve exquisite detail, but must be carefully monitored to prevent damage. If left on for too long, small cracks or damage can occur. If there are thin sheets of calcite on the fossil these might lift or spall. It works by swelling clay minerals, which softens the matrix to the extent it can be washed away. Overpreparation may damage the calcite if small amounts of clay minerals are present. 

Where fossils have details like very delicate spines, KOH can be a viable option if watched carefully. If a fossil has fragile or delicate spines, or an unknown disarticulated nature, mechanical preparation may not be possible. In these cases, it could be that KOH is a good chemical to use. 

KOH is the most vicious of the chemicals we will be discussing here in terms of danger to humans. It therefore should be handled with extreme care, and as with all of these chemicals, please read the safety data sheet. There's a brief list of safety precautions below, but these are not thorough or exhaustive. 

1. As with other forms of chemical preparation, as much matrix should be removed as is possible using mechanical means – working as close to the fossil as you are able. This means the exposure to the KOH is limited and reduces the number of cycles required to get the end result. You may need to do repeat applications in specific areas. 

2. Any cracks should be stabilised using a thin, synthetic resin like one of the AKEMI resins. Cyanoacrylates (superglues) may not withstand soaking in water for a prolonged period, and the use of cyanoacrylates will lead to discolouration. This can be removed afterwards by vigorous scrubbing with acetic acid. Areas that you do not wish to expose to KOH can be coated with Paraloid B-72, wax or another barrier substance that is not water soluble, but removable by other means after preparation is complete. Make sure that the fossil is completely dry before starting. Place it in a suitable dish that will not pose a safety concern during the exothermic (heat-releasing) reaction.

3. Place the flakes onto the fossil with tweezers. Focus on the areas with the most matrix. In many cases, you will be able to just balance the flakes on, but if you have vertical surfaces or need the flakes to stick better, you can give the fossil a very gentle and light spray with water which the flakes will stick to. Because KOH is hygroscopic, it will attract the moisture in the air and 'melt'. You can spray again if needed from a distance so as not to disturb any of the flakes. 

4. Check back frequently to see how your fossils are doing. Every 1-3 hours (sometimes more often) is necessary. You can check to see if the fossil itself is being affected. Most fossils will withstand 4-6 hours, but it is important to check frequently. This is particularly crucial when working with surfaces that have very thin calcite, or areas of thin calcite (common with shells). You can be very precise with where you apply the KOH, so use this to your advantage and only use it where it is necessary. 

6. Once you've finished the a cycle with KOH, thoroughly scrub the fossil with a stiff brush (not a metal brush like brass or copper) in clean water.  Then soak the fossils in either dilute vinegar or water. If you choose to soak the fossil in vinegar first to neutralise the KOH, bear in mind that it might slightly attack the calcite of the fossil. The idea is that the KOH and vinegar neutralise each other, meaning no damage to the fossil, but caution must be taken. 5-15 minutes is usually plenty for soaking in vinegar. Note that distilled water is the more effective in combination with vinegar than tap water. 

7. It's important to remove as much KOH as possible, and to draw it out of the specimen as if left it can cause damage to the fossil in the future. The best place to clean your fossils is to suspend them in your toilet cistern (if you have one). You can use a cloth bag to hold them up, and if it's your household toilet, you're guaranteed a frequent replacement of water! The potassium hydroxide will be gradually drawn out. If this is not an option, or the fossils are too large, you can leave the fossils in running water in a container for a while, and then soak in fresh water for at least 48 hours to a week, changing the water occasionally. A few drops of acetic acid in the water will also help neutralise the KOH. It's particularly important to remove as many traces of KOH as possible, not just for the fossil, but for you as well when you are handling your collection. You can check that your fossil is neutralised by using pH indicating paper. 

8. Once your fossil is dry, you might notice a hazy appearance. If this happens, you can brush on some very dilute acetic acid, leave for a few seconds and then rinse thoroughly. 

KOH must always be stored tightly closed - it is hygroscopic (water-attracting) and also binds the carbon dioxide in the air with the formation of (inactive) potassium carbonate. Do not get KOH confused with Sodium hydroxide, which is otherwise known as Lye or Caustic Soda, often used in soap making or as a drain cleaner. It seems tempting, but doesn’t really work.