Staff at Boulton & Cooper Fine Art were presented with the archives which date from 1890 to 1950 by a builder who was given them by the owner of a house whose garage he was clearing out.
The photo archive provides a vivid insight into life at the factory in York and shows workers on the shop floor, the Rowntrees fire brigade, the company board and staff enjoying physical education classes, among many others.
There are glass plates, slides and photographs in the collection from the company famous for smarties, Aero, Fruit Pastilles, and Kit Kat.
Given the comprehensive nature of the photographs, a guide price of £400 to £600 has been placed on the photographs.
“It is really unusual collection,” said auctioneer George Holtby. “We often get old photographs but to get a collection that is as complete as this is and covering such a long period of time is amazing.”
Also included in the auction are a 1902 Christmas Chocolate Tin and a 1902 Coronation Tin - both of which amazingly still include the chocolate - plus other chocolate tins, as well as a Victorian Group Photograph, from around 1858, which was taken at the time of the marriage of John Rowntree to Elizabeth Hotham, as well as other photographs and glass plates of the Rowntree family.
Mr Holtby said: “We have got things related to the Rowntree family and items related to the workers which tells you a lot about the industry, marketing and the social side of the factory. Back in the day, the bosses did a lot more for the workers than is the case now. They gave them a lot of support.”
*A preview will be held at the Milton Rooms, Malton, on Tuesday, November 25, from 9am to 7pm. The sale will take place the following day.