I have just started with cyanotypes(30-40 printings so far), and I can recommend that you don't try the 175W Mercury Vapor lamps. Their advantage is that they are inexpensive (<$30 for lamp and fixture at Home Depot), but the UV output must be abysmal. My exposure times are 1-3 minutes in sunlight, and 3-6 HOURS! under the MV lamp. In his article, Sandy King mentions that they are "slow", but that is understating it a bit. The higher power units apparently have significantly more UV output, but my next step is to build a bank of fluorescents, and try BL bulbs.