I realised I hadn’t written about this method of knowing what purity of metal you have, despite how important it is in gold and silversmithing, and of course it is relevant to alchemy because it is a simple way of checking if you have made true gold or not.
To put it simply, here is a modern touchstone:
Historic ones were made of dark dense hard stone, such as, according to the note in the Sisco and Smith translation of Lazarus Ercker’s book on assaying, fine grained black rocks such as quartzitic schists or like jasper, or even hard slate.
All you do is drag the piece of metal to be tested across it, and you will get a streak:
The colour of the streak tells you what it is, and like anything else, your judgement improves with practise. In the above example they are, from left to right, lead, lead free pewter (Tin with a bit of antimony and copper) and sterling silver, which is silver with 5% copper. The latter one clearly being shinier and whiter than the others.
showing the number of needles of different alloys used for comparison. That is what makes them such a strong test, the comparison with known alloys. Lazarus Ercker describes the manufacture and use of them, specifying 20 different grades made up of pure gold through different alloys of gold which contain copper and or silver. The method does not appear in Theophilus, who was not so concerned with testing metal purity. In fact as far as I know this method was more used in the late medieval period, and I have not found any evidence for it being older than that, although it probably is. Certainly the silver in Chaucer’s Canon’s Yeoman’s tale is assayed by hammer and fire, not by touch.
Usefully it is also minimally destructive. The older methods of testing involve beating the sample out flat to see how ductile it is, or melting it in a crucible and seeing what colour it turns. This method permits you to test something that has already been made into a nearly finished item, and is surely an often overlooked assistant in the drive towards proper quality control by guilds and thus the rise of hallmarking. Non-destructive testing means that post-manufacture testing is possible and easier to do, rather like the modern situation which uses XRF’s to test areas of the jewellery that is sent for hallmarking.
Hallmarks were introduced in London in 1300, as part of an overhaul of the older regulations used to control the work of gold and silversmiths. Gold and silver plate were to be of fixed alloys, and this was tested by the Goldsmiths company (Who still do that work today) and the mark to be punched on silver items. Later in the medieval period other towns were chosen to act as assaying centres, and 4 still survive, London, Birmingham, Sheffield and Edinburgh.
The proper analysis was usually done by cupellation on a sample scraped or drilled from the finished item, a technology that was fully mature by the late 16th century time of Ercker, but probably not in existence much before the 13th century.
Now, here are some gold look-a-likes:
This one is, from left to right, bronze, brass and gold. I managed to source a tiny piece of purish gold, but the streak it makes is a bit on the red side. Nevertheless, the other two streaks are clearly not properly shiny in the way that real gold would be. And you can clearly see the difference.
Some time in the early modern era the tests were refined to involve the use of acids. Which acid dissolved which streak or changed its colour would define the metals involved, although I don’t know much about this. Either way it continued in use until the 21st century in some assay offices!