Calcium exists in large quantities in the human body, constituting the skeleton as a scaffold supporting the human body, and also plays an important role in secretion, transportation, muscle contraction, nerve conduction, etc. Calcium exists in two forms in the body, one is ionic calcium, which exists in the blood circulation, so-called blood calcium; the other is bound calcium, which is bound to proteins, carbonic acid, or phosphoric acid, and is deposited in tissues, and in addition to the bones and teeth, when normal, calcium permeates in all tissues and cells. Normally, calcium permeates all tissues and cells except bones and teeth, and does not normally appear in a solid state in the tissues. However, in some cases, calcium precipitates into a solid state and settles in the tissues, which is known as pathologic calcium deposition, and the calcium salts deposited are mainly calcium phosphate and to a lesser extent, calcium carbonate. Calcium salts are usually monorefractive, but calcium oxalate is birefractive. Calcium is usually purplish-blue when stained with HE, and many dyes can form chelating complexes with calcium, including alizarin red S, red violet, and nuclear fixation red, etc. Alizarin red S is an anthraquinone derivative, the sodium salt of alizarin sulfonate, which chelates with calcium salts of calcium carbonate or calcium phosphate to form an orange-red complex.

Calcium salt staining solution (silver nitrate method), is a metal replacement method, the principle lies in the Von Kossa silver solution on sections containing insoluble calcium salts, calcium is replaced by silver, silver salts in the role of light is reduced to black metallic silver, suitable for a large number of samples of calcium salt tissue staining.