The majority of the alternative techniques and historical processes are not so sensible to light and it is therefore impossible to use an enlarger as on traditional fiber paper prints. Negatives are printed with contact print technique.
If there’s even a minimum space between paper and negative, it could be the littlest – it doesn’t matter, the final image – at best- won’t have the finest details and the micro-contrast, will be blurry and soft. When the space augments, there will be completely blurred zones, particularly bothersome and impossible to hide.
This happens when printing on papers that tend to whiten and undulate a lot. They have depressions where negative is detached from paper that are translated as regular bands, alternatively flou or clean. That’s the reason why one of the most useful characteristics of a certain type of paper is flatness even when wet. A wonderful cyanotype paper, Shoeller durex, is practically unusable because of this problem. Before the choice of the support, it is appropriated to verify that our paper will be smooth even when sensitized.
The real solution for this problem is using a a high pressure printing frame. This way, all the depressions of the paper are completely pressed against the negative, the contact is complete on the entire image surface and prints are perfects in every point.