Congo Red staining

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Originally used as textile dye, but has since superseded by more modern dyes. Documentation from the time of its original naming is lost, but it is thought that the name was chosen around the time of the West Africa Conference, a summit involving several European colonial powers of the time, ostensibly over trade in Africa, particularly the Congo River basic area, but perhaps with a subtext of how to divide up territory in Africa. Although Germany was not a significant colonial power, the conference would have been topical at the time and the name may have been chosen to conjure up images of colourful African dress (although clothes from the Congo region were traditionally dyed black).[1]

Diazo chemical. Used primarily in the staining of amyloid. Materials that an affinity for Congo Red (CR) are known as congophilic.

The amorphous amyloid stains red.
Under polarised light, amyloid shows a mainly green birefringence

The most well known congophilic material is amyloid and also shows birefringence under polarising light, typically 'apple green', but other colours may be seen.[2] CR also avidly stains eosinophils. Depending on the exact method of staining, collagen and elastic fibres may also stain positively.[3]