Trial art

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Tasters in front of a tasting oven

In metallurgy, the art of tasting or docimastics refers to the knowledge and application of means and procedures in order to quickly determine the ingredients of minerals and products from smelters. The art of experimentation is the forerunner of analytical chemistry.

Basics

The art of tasting is an ancient science. With the help of various methods, the fineness of precious metals in an ore sample was determined. In ancient times, mineral acids were not yet known, so the base metals were separated from the noble metals by means of a melting process. In this dry chemical method, gold-bearing ore was provided with a lead-containing disintegrant and melted down. The lead was then evaporated in a blowing process and the gold content of the sample was determined gravimetrically. This quantitative method is already mentioned in several places in the Old Testament. Other methods of determination were later developed and used.

The art of tasting was used in the European Middle Ages and the early modern period by state officials, the Wardeins . The principles and rules of the art of tasting were entered in a tasting book which the tasters kept. The tester often carried out the individual examinations in a test house. As a reward, the tester received the trial fee.

Workflows

The work involved in trying out was carried out in several steps. First of all, the tester took a sample (test product) from the product to be checked. The sample was prepared by further preparation steps such as drying, crushing or sieving. The sample was then weighed; this weighing was carried out several times in the workflow. Next came the analytical operations which, in the early days of the art of tasting, were carried out dryly in the tasting oven. Wet analyzes using chemicals were also performed.

Tools and equipment

Tasting vessels
Trial needles

For the application of the art of tasting, the tester had a number of tools and devices available, which were used depending on the work step. These were the tasting bushel, the touchstone , the tasting nobility , the tasting scale , the tasting weights planchet, the tasting gulf that Probierschirbel, the tasting oven and the tasting plate.

The trial bushel was a container with a precisely determined volume, this measure was introduced at the Joachimsthaler Hütte. The sampler filled the ore samples taken from the ore heap into the sampling bushel.

The touchstone or string stone was a black, not too hard stone, which did not foam when poured over regia and was not attacked by the nitric acid. The stone was used to determine the degree of mixing of the metal piece. For this purpose, a line was drawn on the stone with a test needle and the piece of metal to be examined. The tester could draw conclusions about the metal composition from the similarity of the line color.

The respective weights of the samples were determined with the test balance . Each tester had at least two tasting scales. A scale served as a weighing scale to weigh in the tasting flour. The other set of scales was used as a grain scale to weigh the seeds that were applied. Some tasters also had a third set of scales to weigh the lead weights.

The sample weights were weights of different weights that were used for weighing. There was the common trial weight, the mark weight and the pfennig weight, which was also called Richtpfennig. There was also the tasting centner, which was a centner divided into several equally heavy partial weights.

The sample bowls (sample bowls) were small bowls made of copper that were placed in the weighing pan of the weighing scale. The test flour was poured into these bowls with a spoon.

The trial gear was a kind of forceps with a reset mechanism. The trial gap was pushed apart again by spring force after it was pressed together. With the trial joint, the tester placed the respective sample pegs in the trial oven or took them out of the oven.

The sample pots or sample bowls are small refractory vessels into which the sample flour mixed with additives (sample lead) was filled. The sample pots were then placed in the sample oven until the sample flour slagged from the heat of the fire.

The tasting oven was a small oven made of brick or thick sheet metal. In the furnace, the respective samples were melted and slagged.

The sample sheet was a board made of copper or iron. The slagged samples were poured into the test sheet and, after cooling, crushed with a stick.

Auxiliary materials

Various corrosive liquids such as hydrochloric acid , nitric acid , acetic acid , wine vinegar and aqua regia were used as auxiliary materials. But carbonic earth, potash and rosin were also used. Salts were used to dissolve, and tartar was also used to dissolve limestones . Sulfuric acid was required to dissolve iron or zinc . Gold was attacked by aqua regia. Common table salt, borax , glass bile , ammonia and saltpeter were also used.

sampling

Sampling was the first step in trying. Depending on the material to be examined, the steps involved in taking samples were structured differently.

Ore samples

It was important that the tester took the sample from the existing pile of ore in such a way that the sample represented a representative cross section of the total amount. In the case of heaps of fairly similar composition, e.g. B. iron ores, the sample was taken from the center and from several points of the edge area. Difficulties arose when the material to be checked was available in different compositions and grain sizes. Several samples were taken here, the pieces were then crushed, mixed together and then a sample was taken from them. There were precise instructions for the preparation of such samples.

Metal samples

There were four methods of sampling for metals:

  • Excavation test
  • Drill sample
  • Chip test
  • Sample

The excavation test was mainly used for cast bars such as silver bars or gold bars. For this purpose, the surface of the ingot to be tested was first cleaned. Then a so-called cut was made with a gouge at several points on the ingot. The knocked out pieces, the cut, were then flattened out to thin leaves on a polished anvil . Small pieces were then cut off from these leaflets with scissors and melted down.

During the drilling test , the metal piece was drilled through in several places with a drill. The resulting drilling chips were then melted down and the molten metal was then poured into thin strips on a bare iron plate. After cooling, the strips were cut into small pieces. The procedure was used for copper and lead.

For the chip test , a polished iron rod was dipped in molten metal, mostly copper, and the attached metal crust was then chipped off and laminated. However, this method was less suitable for taking samples.

The scoop test was used for molten metals . It was used with alloys and, when used correctly, provided very precise information about the composition of the total amount.

Sample preparation

After the sample was taken, it was prepared for analysis. A part of the ore sample was first cut off, which the sampler kept for further investigations. The remainder of the ore sample was first dried in iron pans and then divided into five portions, after which they were sealed and shipped. The chief mining office , the smelter, the trades tester , the smelter and the ore supplier each received a portion of the sample .

As a further step, the moisture content of the sample available for further investigation was determined. For this purpose, part of the sample was taken from the center of the vessel and weighed. This part was then dried and weighed again; the moisture content of the total sample was determined from the difference. The entire sample was then dried. For this purpose, the sample was filled into a bowl and heated either in a water bath to 100 degrees or in an air bath to 120 degrees. There were special drying discs for several samples. After drying, the sample was finely ground in a mortar and then sieved through a brass wire or hair sieve. Any cast iron particles present were removed from the sample flour with a magnet. Then contaminating substances were leached out in a safety trough. In the case of metal samples, the preparation of the sample was limited to flattening and crushing the metal strips.

Analysis of the sample

Two basic methods were available to the tester for a precise examination of the sample. These were on the one hand dry chemical methods and on the other hand wet chemical methods. The sample for initial results also served as a quick test for metal samples .

Dry chemical methods

With the dry chemical methods, the sample to be examined was mixed with additives such as alum, borax or glass powder and then placed in a fire-proof test crucible. The sample was then strongly heated in a sample oven. To determine the amount of aggregate , the tester placed a small pinch of ore on a shovel and held it in the fire. Based on the color of the smoke that the ores developed, the tester could then see what amount of additives had to be added. The further treatment of the ores was very different and depended on which metal was present. Ores of precious metals were mixed with sample lead, melted down, and after melting, the lead was driven out of the alloy by reheating.

Wet chemical methods

In the wet chemical methods, mineral acids were used in order to be able to examine the silver or copper-containing alloys obtained during the blowing process in more detail. Depending on the type of acid, the metals were attacked to different degrees by the acids. The tester made use of this knowledge in the wet chemical methods.

Solder tube analysis

A special examination method is the solder tube examination. When analyzing with the soldering tube , precise statements can be made about the composition of a sample. However, this method cannot be used for all samples. With the soldering tube analysis, conclusions can be drawn about the composition of boron and copper compounds based on the flame color. Investigations into the fusibility of the respective metals can also be carried out with this analysis method. With the solder tube examination, the composition can be determined at lower temperatures based on the behavior of the respective sample.

Coinage metals

Trying the coin metals has a particularly high priority in the art of tasting. Most of the coins were made from precious metals like gold or silver. Even minor deviations in the precious metal content changed the value of the coin. For this reason, the coin metals were checked with particular care. There were special tasting regulations for tasting the Saxon denarii, thalers and groschen. The coins were weighed and melted down, then the metals were divided into the individual fractions. Gold was separated with separating water, copper and lead were volatilized by heating.

literature

  • Newly opened tasting book. In it not only all the secrets of the art of tasting, the dressing and figuring of the Ertze, the melting of the same and a few chemical hand movements are discovered, but also how everyone learns this noble art without any other oral instruction from beginning to end, is taught . Rüdiger, Lübeck 1744, OCLC 312870908 . ( Digitized edition of the University and State Library Düsseldorf )
  • Theodor Richter ( arr .): Carl Friedrich Plattner's probing art with the soldering pipe . 4th edition. Publisher by Johann Ambrosius Barth, Leipzig 1865. (digitized version)
  • Bruno Kerl: Th. Bodemann's instructions for the art of mining and smelting . Completed and partially revised, second edition. Verlag der Grossesche Buchhandlung, Clausthal 1856.

Individual evidence

  1. a b Theodor Bodemann: Instructions for the art of mining and smelting . Verlag der Schweizerische Buchhandlung, Clausthal 1845. (online at: Bayerische StaatsBibliothek digital )
  2. ^ A b c Karl Heinz Koch: The automation of an ancient method. (last accessed on February 20, 2013; PDF; 302 kB)
  3. a b c Johann Christoph Stößel (Hrsg.): Mining dictionary. Chemnitz 1778. (online at: Bayerische StaatsBibliothek digital )
  4. ↑ The art of tasting. In: Meyers Großes Konversations-Lexikon. Volume 16. Leipzig 1908, p. 363. (online at: zeno.org ) (last accessed on February 20, 2013).
  5. ^ CE Gellert: Johann Andrea Cramer's beginnings of the art of tasting. Publishing house of the Heinsiussische Buchhandlung, Leipzig 1766.
  6. a b Bruno Kerl: Metallurgical trial art for use in lectures and for self-study. Published by Arthur Felix, Leipzig 1866.
  7. ^ Georg Agricola: De re metallica libri XII. 1556. (Latin) ( Twelve books on mining and metallurgy. VDI-Verlag, 1928, DNB 579073963. - Reprint: Marix-Verlag, Wiesbaden 2007, ISBN 978-3-86539-097-4 ).
  8. Gerhard Ackermann: Analytics with the soldering tube. (last accessed on February 20, 2013; PDF; 100 kB).
  9. Peter Hammer: Trying the coin metals. (last accessed on February 20, 2013; PDF; 468 kB).

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