The “white spots” that brought the cultural relics back to life are actually plaster?

When visiting museums, we often see a large number of unearthed cultural relics, many of which are vivid pottery pigs, cows, horses and people. There are always some white "spots" on them. People can't help but wonder what these white spots are. In fact, these are traces of plaster left after the cultural relics have been restored.

Image source: Pixabay

What? Plaster? Really?

It is true. Currently, the materials used for pottery repair are mostly medical ground gypsum powder, which is the kind of medical plaster used to quickly fix fractures. It is made by low-temperature calcination and dehydration of natural gypsum to form white powdered gypsum (chemical formula: CaSO4·1/2H2O, with inert acrylic resin added during the production process). The main difference between it and raw gypsum (CaSO4·2H2O) is the water content. When it comes into contact with water, gypsum hardens quickly to become raw gypsum. Therefore, we can use molds to shape the desired shape to complete the pottery repair work. Due to the reasons such as easy access, non-toxic and harmless, widespread use, non-expansion, high strength, easy trimming, and relatively simple shaping, etc. [1], gypsum is widely used in the restoration of pottery cultural relics.

Then someone will ask, why not paint these plasters in a color similar to the cultural relics? This is because the restoration of cultural relics emphasizes the principle of repairing the old as it is. Since the plaster used to fill in the damaged parts is mostly white, and the color difference with the artifact itself is large, if it is used for display, some of them will be painted to make the artifact look old. The principle of making the artifact look old is to repair it to look like the original, but not like the original. The pigment used to repair the old should be coordinated with the original color of the artifact, but not completely consistent with the color system, so that the color difference of the artifact is not too large, and it is not easy to see from a distance. If you look closely, the traces of repair should be distinguishable. However, gypsum products are not suitable for coloring, or in other words, the color can only be used temporarily, and the lifespan is not long. Repeated use of gypsum materials for repair will lead to further damage to the cultural relics. Therefore, the pottery cultural relics we see have to leave a large number of white "spots".

References:

[1] Huang Xianyuan. On the application of gypsum in pottery restoration[J]. Art Science, 2015, 28(08): 111.

Source: Chongqing Science Writers Association

Author: Zou Qinjia, Class of 2021, Cultural Relics and Museums, Hebei Oriental University

Audit expert: Li Hanbin

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