Ambrotype c1850 - 1890
Ambrotypes are an underexposed glass negative which has been painted on the back with black shellac paint. The image then appears as a positive which can also be hand coloured. They are usually framed to protect them with a glass cover.
Please do not try and clean an Ambrotype. In some cases the glass cover is not present due to damage and this means the image surface is exposed and can very easily be wiped away completely. They are repairable and recoverable though if done carefully.