The nineteenth century saw a great rise in interest in history, which was probably a Good Thing but, on the other hand, it also saw a lot of slipshod, nationally-biased work where authors either fabricated evidence, or were very careful with what they selected.
Nineteenth century craftspeople and artists were also very fond of the romantic ideals of the middle ages, and produced many well-known artworks that are widely believed, by the uninformed, to be from the middle ages. The Pre-Raphaelites and the later Craftsman movement both drew inspiration from medieval and renaissance sources.
An example of an artwork of this type, and why it's a bad source for our purposes, is discussed here: http://jauncourt.i8.com/accolade.htm
Using 19th Century sources in the SCA
Generally, if something is a nineteenth century work, don't use it unless
- You really know what you are doing, or
- It's a nineteenth century reprint of a historical document.
While it is not unknown for historical documents to have been fabricated in the 19th Century, you are more likely to be on solid ground if you use 19th Century document collections (eg the Historical Manuscripts Commission collections) than if you use nineteenth century interpretations (eg Burckhardt).
A similar principle applies to using 19th Century copies of medieval artworks. The general outline of the picture is usually copied correctly, but the smaller details often have errors which may be misleading (e.g. giving the idea of corsets under bliauts). Ninteenth century copies can give you an idea of whether you might want to look for an original (if it still exists), but often less effort and misconceptions are involved in looking up an original copy in the first place. Many internet sites redistribute 19th century copies of illuminations (without warning about potential errors), because unlike modern accurate copies, 19th Century copies are out of copyright.