A flawless wine is clear, this can be determined especially with a white wine by holding the glass against a light source. This is difficult with dark red wine. Cloudiness in the form of clouds, haze or streaks are called blind in wine evaluations and often indicate a more or less pronounced wine defect. In most cases, this is caused by improper wine-making methods or a lack of hygiene. The manifold reasons can be of a physical-chemical nature (proteins, tannins, crystals, metals) or of a biological nature (bacteria, yeasts) or are caused by substances foreign to wine such as dust, filter material, cork abrasion and the like.