The purpose of making Plaster of Paris and charcoal tablets is for producing visible effects when metals, rock chips or crushed minerals are exposed to the oxidizing or reducing flame generated by the blowpipe.

The following images are intended to aid the prospector better understand what is contained within old blowpipe books and to begin using the blowpipe with these inexpensively made tablets.

The 1st image shows the box of Plaster of Paris found at Wal-Mart for about 3.00 and a portion of the dry contents in a steel bucket, which, when an appropriate amount of water is added and quickly stirred a thick slurry is formed. The 2nd picture shows this slurry poured onto a piece of glass, which is to be spread out as far and as fast as possible, yet maintaining a thickness of at least ¼ inch. The stirring should be as quickly as possible to both mix the water and plaster and to remove as many air bubbles as practical. After a couple batches have been made one has become an expert.

Plaster of Paris begins to set or harden almost as soon as water is mixed with the powder, so speed is very important. To avoid making errors and waste of time and material start by using a little of the powder mixed with water to determine amount of water, time to stir, empty the bucket and place slurry on glass. The glass serves to provide a smooth flat surface to spread the slurry, as well as make a flat surface to one side of the plaster. Don’t worry about the plaster adhering to the glass, gentle bumps to the mass will dislodge it.

Depending upon how thick the cake is made this box of this plaster makes about 20 to 30 of these 2x4 inch tablets. Sometimes, I make 1x2 inch sizes for preliminary tests so as to not use up the larger tablets. Making larger tablets has the advantage of capturing all the sublimates, whereas the small tablets will not always allow all the oxidation to be contained within the small area, which can cause potential confusion by not seeing all the oxidation results.

Besides working with Plaster of Paris there are charcoal blocks, sticks and tablets that the blowpipe analysis utilizes. I have not been able to find sources of charcoal blocks or suitable sized sticks, so I elected to make them. The next image shows a bag of wheat flour, a ceramic mortar/pestle, a jar of activated charcoal granules and a 2x2 inch resultant charcoal tablet.

This inexpensive activated charcoal from Wal-Mart (fish dept) is pulverized to a powder in the clean mortar. Then the charcoal is mixed with an equal amount of wheat flour to which is added enough water to form pasty dough balls, which are then flattened out upon a sheet of glass to form a flat surface and allowed to dry enough to be safely removed. Both the plaster and charcoal tablets must be allowed to thoroughly dry (2 to 5 days) before being used.

The below charcoal tablets are approximately 1” x 2” and about ¼” thick. They can be made as large as desired, but if larger and thicker requires a lot of extra grinding of the charcoal grains to a powder. I use wheat flour because it is a great binder and becomes charcoal when burnt. So, before using these tablets I always use the propane torch flame to burn the surface, which converts the flour to charcoal and the un-burnt flour continues to bind the tablet. These charcoal tablets have advantages over the white Plaster of Paris because of the black color, which can expose subtle oxidation colors (sublimates), as well as aid in reducing some minerals to metals

Pieces of lumber can and do act in the same manner as the homemade charcoal blocks, but a lot simpler and easier to fabricate. I prefer using 1”wide x 2” long x 1” thick pieces of pine, fir and oak that are burnt evenly on one side to form inexpensive charcoal blocks. The above picture of a block of pine appears bluish, but it is really black. The old books say to use soft woods for these types of charcoal blocks, but I have found that pine is too soft and buckles under the heat when blackening the surface, which creates cracks and an uneven surface. Experimentation is an essential key in saving money, while at same time producing verifiable results, so examine the possibilities that abound around the home.

For the most part these plaster & charcoal tests are saved so that I can document with pictures, as well as keep them in boxes for future reference. Tagging each tablet or piece of wood to identify the experiment is essential, which is basically the same thing as labeling collected rock samples in the field, because if they are not tagged as to exact location they were extracted the samples become of questionable value for future study.

Another important issue is about why the tablets should have smooth flat surfaces. By being flat and smooth the hot blast from the blowpipe will spread the fine oxidized powder evenly outward away from the test site, which will display both color and content that will be scrutinized. These sublimates provide the clues that will be needed to how to distinguish the various metals and metals within mineral matrixes. Yet, and because of these smooth flat surfaces they are not particularly amenable to the blast of air from the blowpipe. So, to avoid the rock chip or the pulverized mineral from being blown away or scattered upon the tablet a carved out depression is made into the tablets to help contain the sample near the end of the tablet where the test will be initiated.

The next step on this simplistic journey is to visit the metal sublimates page which have been produced by the common hand-held propane torch on Plaster of Paris tablets.