Citing archival sources may be different depending on the ciatation style you are using (for example Chicago or APA).
The citation might take the following format:
[identification of the item] courtesy Clemson University Libraries' Special Collections and Archives.
We recommend you visit the Citation Styles: Formatting & Style Guides - Research Guide.
Citing materials from manuscript collections and the University Archives
Use the following format:
Identification of the item, folder and box number, and title of the collection, Special Collections and Archives, Clemson University Libraries, Clemson, South Carolina.