vinegar. The deterioration process is therefore known as "vinegar syndrome." The negatives become very brittle and, in diacetate and triacetate film, the base shrinks, causing grooves ("channeling"). In addition to fading, silver-based images are prone to silver mirroring, which presents as a bluish metallic sheen on the surface of the photograph or negative and is caused by oxidation, which causes the silver to migrate to the surface of the emulsion. Color photographs are an inherently unstable medium, and are more susceptible to light and fading than black and white photographic processes. They are composed of various dyes, all of which eventually fade, albeit at different rates (causing discoloration along with fading). Many color photographic processes are also susceptible to fading even in the dark (known as "dark-fading"). There is little that can be done to restore faded images, and even under ideal conditions, most color photographs will not survive undamaged for more than 50 years. Preservation strategies Temperature and relative humidity control Maintenance of a proper environment such as control of temperature and relative humidity (RH; a measure of how saturated the air is with moisture) is extremely important to the preservation of photographic materials. Temperature should be maintained at or below 70 °F (the lower the better); an “often-recommended” compromise between preservation needs and human comfort is 65-70 °F (storage-only areas should be kept cooler). Temperature is the controlling factor in the stability of contemporary color photographs. For color photographs, storage at low temperatures (40 °F or below) is recommended. Relative humidity should be maintained between 30-50% without cycling more than +/- 5% a day. The lower part of that range is best for "long term stability of several photographic processes". Not only do relative humidity levels above 60% cause deterioration, but also low and fluctuating humidity may also damage them. Climate control equipment can be used to control temperatures and humidity. Air conditioners, dehumidifiers, and humidifiers can be helpful, but it is important to make sure they help instead of hurt (for example, air conditioning raises humidity). Cold storage Cold storage is recommended for especially vulnerable materials. Original prints, negatives, and transparencies (not glass plates, daguerreotypes, ambrotypes, tintypes, or other images on glass or metal) should be placed in packaging (archival folders in board boxes in double freezer weight Ziplock bags) in cold storage, and temperatures should be maintained at 1.7–4.4°C (35–40°F). According to the guideline of National