touchstone(shì jīn shí)

touchstone

This entry is reviewed by the project of "science popularization China" science encyclopedia entry compilation and application.

A kind of black hard stone, draw a stripe on it with gold, you can see the fineness of gold. The mineralogical name is basanite, also known as chert slate, siliceous slate, etc. It belongs to basaltic extrusive rock, dense, hard and black. As an ornamental stone, the whole body is dark, and if there is gold thread on it, it is the top grade. The shape of the touchstone is different. Some are round as an egg, some are full of wavy pits, and some leave marks like the same person's fingernails.

It is an accurate and reliable test method and basis.

unearthed relic

The picture on the right is a stone axe from the Qijia culture in Gansu Province. The stone axe is dark in color. When gold is used on the stone axe, a yellow scratch will appear.

Mineral composition

Touchstone is a kind of stone which can be used to identify gold. Its mineralogical name is basaltic rock. Most of the stones used as touchstones are dense and hard black siliceous rocks, such as siliceous slate and chert. Chert rock is composed of protein, chalcedony and microcrystalline quartz. It is dense and hard. The shell fracture is carbon or black, and it is often layered, banded, convex mirror or nodular. Chert banded or nodular often occurs in carbonate rocks. The dark chert pebbles near guzong gold and copper site in Xinjiang, the aeolian dark chert with prism commonly known as "desert paint" on the Gobi desert and the black Yuhua stone in Nanjing can be processed into good touchstones after grinding.

Testing gold

In ancient times, the authenticity and fineness of gold were identified by carving on the touchstone. In the traditional gold identification method, there are a series of gold medals that can be compared with each other. It is more rigorous and reliable than the simple methods of weighing, listening, nail depiction and fracture observation. Therefore, even in modern times, it is still one of the main identification methods for professional appraisers of some banks and gold shops. However, driven by the development of science and technology, compared with the accurate methods such as electron probe microanalysis and X-ray fluorescence microanalysis, the touchstone method, which relies heavily on the discerner's eyesight, has shown a disadvantage in terms of accuracy and began to withdraw from this stage. the color of gold powder engraved on the touchstone is called streak in mineralogy, and it is also a common method in modern mineralogy to distinguish minerals by the color of mineral powder (streak). Gold is soft and easy to streak. The darker the touchstone is, the smoother and smoother it is, and the clearer the marks are. It is usually made of black chert or siliceous slate with a hardness of about 6.5 and fine texture. The principle of distinguishing the fineness of gold with touchstone is to judge according to the reflectance and color of streaks. Therefore, there is a formula of "looking at the color horizontally, looking at the light obliquely", which is a colorimetric method. The gold sample is scratched on the touchstone, and then a series of gold medals with standard fineness are scratched on the same touchstone. The main color of the gold medal is viewed horizontally and the floating light is viewed obliquely. The comparison is made visually until the gold medal is consistent with the sample. At this time, the fineness of the gold medal is the sample fineness. This kind of colorimetric operation must be carried out by a master with rich practical experience, and due to the different colors of clear color gold, small mixed gold and large mixed gold, it is necessary to select clear color gold, small mixed gold and large mixed gold respectively for gold medals. Choose the right series of gold medals, but also from 12 to 24K a total of more than 20 gold medals one by one comparison. After all, with the development of science and technology, the composition of gold ornaments is becoming more and more complex. The touchstone detection method, which accounts for a large proportion of human factors, can no longer deal with the problem of high precision and high confusion with a limited number of gold medals.

Assay method

Try to be clean

It's better to use clean water when grinding and washing, and wash it with clean water after grinding, and keep the stone surface clean before and after applying oil. Take both ends of the stone. Don't touch the stone surface with your fingers. Be careful not to be contaminated with dust and damp (especially the air in your mouth and sweat in your hands). Otherwise, it's not easy to stain. The oil used should also be clean. The shell of Castor seeds should be removed. The seeds should be wrapped with silk cloth and crushed to make the oil leach from the cracks of the cloth. When the oil bag is not in use, it should be placed in a bottle or box with a cover to prevent dust contamination.

Apply oil evenly

When applying the oil, it should be applied evenly. The stone surface should be divided into two parts. The oil passage should be 2 cm (CM) wide from both ends of the touchstone. The edge of the oil passage should be straight and parallel to the edge of the touchstone. Form a sharp boundary with the other half that has not been smeared, so as to make the length of the gold channel consistent.

The oil level should be flat

After applying oil, wipe off the oil slick with clean silk cloth, so that the oil is a thin layer, but it should not be too dry, too dry is not easy to stain, too thick is easy to roll oil and black. When gold is scratched on the touchstone, it will be colored and marked. "Flat color, oblique light, listen to the sound.". Gold contains more silver and is soft, showing cyan on the touchstone; gold contains more copper and is hard, making a sound on the touchstone. If there is a standard gold medal, compared with that mark, it is clear. when using the touchstone to grind the road, generally hold the stone with the left hand and the gold with the right hand. Hold the stone with the thumb on the top and the remaining fingers on the bottom, with half of the oil on the top. Hold the stone horizontally in the hand and keep it stable on the table. When grinding the road, you should hold the material object or the card tightly against the stone surface, and use wrist force to hold the right hand of gold. The road is generally 2 cm long and 0.5 cm wide. The suitable grinding times are about 20 times for large pieces (bars and ingots) and more than ten times for small pieces. When grinding, it should be spread out step by step from one end in order to avoid repeated coating and grinding. The length and width of the real track and the opposite track should be consistent, so as to compare the color.

Extended meaning

It refers to the examiner who tests whether something (theory, material object, skill) reaches a certain height.

Word application

The fourth fold of Jianfu stele written by Ma Zhiyuan in Yuan Dynasty: "here, there is a touchstone." The second chapter of CAI Dongfan's the romance of the Qing Dynasty: "since ancient times, heroes may not be able to make great efforts without the disturbance of their mind. Therefore, the invasion of the enemy and foreign countries is a touchstone to sharpen the heroes." Famous saying: suffering is the best touchstone of life.

Chinese PinYin : shì jīn shí