Bleaching also allows the use of subtle pastel dyes. Best results are obtained when the material is bleached fresh, although some, such as grasses, can be bleached after drying. Certain plants are quite difficult to bleach and a degree of experimentation is often required to obtain an acceptable product. An appreciation of the underlying principles involved in bleaching is necessary to produce high quality bleached ornamental plant material and to avoid the hazards associated with the use of potentially dangerous bleaching chemicals. The aim of bleaching is to remove all coloured compounds.
Oxidative bleaches act by breaking down the coloured compounds into smaller, colourless compounds.
They are the more efficient bleaches, although they cause structural damage to the plant material by breaking down lignin. Hypochlorite and peroxide can also decompose cellulose fibres under extreme conditions. These bleaches require careful use to avoid a brittle and weak final product. Reductive bleaches do not degrade the coloured compounds, but instead chemically modify them into colourless compounds. These bleaches do less damage to the plant structure, but when used alone cause the plant material to yellow with age, which is a major drawback.