Bleaching plant foliage

Page last updated: Friday, 9 December 2016 - 1:58pm

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Bleached ornamental plant material provides a striking contrast when arranged with dried or dyed flowers. Bleaching also allows the use of subtle pastel dyes. Best results are obtained when material is bleached fresh although some such as grasses, can be bleached after drying. Certain plants are quite difficult to bleach and experimentation is often required to obtain an acceptable product.

An appreciation of the principles involved in bleaching is necessary to produce high quality material and avoid the hazards associated with the use of potentially dangerous chemicals.

Bleaching theory

In the following it is the responsibility of the user to apply all necassary caution and protective equipement for helath and safety when using chemical to bleach plant material.  

The aim of bleaching is to remove all coloured compounds. This can be achieved in two ways:

  • oxidative bleaching  (hypochlorite, chlorite, peroxide)
  • reductive bleaching (sulphites, borohydrides).

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 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.

Plant composition

The amount of water, cellulose, waxes, oils and pigments in plant material affects the bleaching treatment.

Water

Up to 97% of most plants is water. Water is present in all parts of the plant and in some cases is responsible for structural support. In these cases, such as the Geraldton wax flower, bleaching is not successful.

Cellulose

The second most abundant material in a plant is cellulose - the 'plant skeleton'. The cellulose fibres which provide structure are embedded in coloured lignin. The fibres may be damaged if the bleaching conditions are too severe. An ideal bleach destroys everything else but leaves the cellulose intact.

Waxes and oils

Waxes or oils often coat the plant surface, where their primary function is to prevent water loss. Unless removed by soap or organic solvent they will prevent penetration of the bleaching chemicals. Proteas need special care in this regard.

Pigments

Chlorophylls (green), anthocyanins (red or blue) and carotenes (yellow, orange or red) are normally found near the surface of the plant. They are easily removed by organic solvents or oxidising bleaches. Lignin (brown) is the pigment most difficult to remove. Complete removal of lignin through bleaching (especially using oxidative chemicals) leaves a brittle product.

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Contact information

Kevin Seaton
+61 (0)8 9368 3244