At first glance the idea of drilling a hole in stone with fruit or vegetable juices may sound completely insane, but from archeological evidence it is possible that our ancestors knew this secret for the past 25,000 years. Man has always been curious about ways of doing different kinds of jobs the easy way; this is one of them.
It is not known who came up with this method of making holes in stone, but it appears that some Witchdoctor who was familiar with making potions from different extracts he made from plants discovered the process. One of the advantages of using the process is that it leaves a smooth hole through a stone. Conventional drilling leaves ridges on the sides of the drillhole.
The naturally occurring acids used are citric and oxalic acids that are made from plant sources. The citric acid is found in lemon juice. The oxalic acid is found abundantly in the leaves of the rhubarb plant. Citric acid is procured by squeezing the juice out of a lemon. Oxalic acid is leached from macerated rhubarb leaves in water. Both of these acids are dilute and have to be concentrated by boiling off some of the excess water to make a stronger acid.
To use these acids for drilling through rock they are dripped into a shallow depression that is created in the rock so it is filled with the mixture of these two acids. This is important because the products of oxalic acid are insoluble. The citric acid products are soluble and will keep the products of the oxalic acid in suspension. They are left in the depression until they stop reacting with the rock, then any sludge in the hole is removed with a spatula. (A stick is just as effective.)
The process is repeated until the hole is as deep as it should be, or it reaches the other side of the stone. A test of this technique on marble resulted in a hole that was 7.5 cm within 15 minutes. When the process is complete the stone should be rinsed with plain water to stop any further reactions.
For more information about this process go to www.geopolymers.org This is described in the section of downloadable papers under archeology.