Fuji Velvia, ISO 100, cross-processed as C-41, Photo by Angela Kleis
I always like to keep a few rolls of color film, both negative and positive, in the refrigerator, just in case. One thing I sometimes like to do when I shoot slides, particularly with certain cameras, is to have them cross-processed as negatives.
Cross-processing, or X-pro, is a technique that involves processing (developing) one type of film in the wrong chemicals, most commonly developing E6 slide (positive) film in C-41 chemicals meant for color negative film. Switching up the chemistry in this way results in color shifts with higher contrast and saturation, which are even more off-set by the presence of any light leaks in the image. The results are unpredictable and vary based on the film brand and film type, as well as the camera used, the subject, and the exposure conditions. Bright colors outdoors on sunny days have produced the most interesting results for me.
Fuji Provia, ISO 400, cross-processed as C-41, Photo by Angela Kleis
The reverse can also be applied to color negative film processed in E6 chemicals, which generally yields flat, muddy images with little contrast resulting from the orange-colored base of color negative film. Over-exposing the film by one to two stops and then pushing the film another one to two stops during processing can help to overcome the fog and increase contrast, if you're feeling experimental. Be sure to explain your needs to the processing lab and be prepared to pay extra for both pushing and cross-processing.
Next time you have an extra roll of color slide film and feel experimental, consider brighter, more colorful subjects and ask the lab to cross-process. The service usually cost a few dollars more and the results are fun, unexpected, and a bit of a surprise - just like film, itself.