Bleached fabrics after wear and washing lose whiteness and often get a yellowish tint. To counteract this yellowness, its complementary colour, blue is used and the whiteness is restored. Hence, blue is used in the last rinses for bleached cotton and linen. It is obtained from chemicals- vegetable and mineral sources and in the form of powder, liquid, balls and cubes. The colour varies according to the sources from violet to blue or from greenish blue to bluish green. They also differ in their solubility. Blues used for laundry purposes fall into two classes:
1. Blues that are insoluble in water:
Ultramarine and Prussian blue comes under this category.
(a) Ultramarine Blue:
This is the most commonly used laundry blue. It gives a violet blue colour and makes a fine powder and thus becomes a suitable blue for laundry. It is safe to use as it is not harmful to the fabrics. It was originally a mineral substance but now it is known to be manufactured from soda ash, sodium sulphate, charocoal, sulphur and clay.
(b) Prussian Blue:
This is a mixture of iron sulphate and potassium Ferro cyanide. It is not suitable for laundry, as it tends to leave rust marks on the fabric on ironing.
2. Blues that are soluble in water:
These are actually dyes and have the advantage of being easy to prepare, control and apply. Since they produce an even colour and leave no sediment, they are widely employed in large scale powder laundries. These blue can be obtained in concentrated solutions or in powder form- purplish blue being the most popular shade, as it gives a whitish appearance. These are the dyes that have a great affinity for materials and must be used with care. However, owing to their great solubility, they are easily removed by thorough rinsing and correction for over-bluing is not a trouble.
The Bluing process
Bluing is to be done only when the fabric is free from soap or detergent used for washing the cloth. The process therefore is followed in the last rinse and the steps are:
1. The blue is tied in a piece of muslin and squeezed in cold water until the required depth of colour is obtained.
2. Ultramarine blue being not insoluble in water, the colouring matter is held in suspension, and so the water must be stirred each time before use.
3. The article is dipped up down in the solution once or twice in such a way that the water is not retained in pockets or other bag-shaped parts.
4. Articles should not be allowed to remain in the bath, but must be moved about all the time.
5. Bluing and starching process may be combined whenever necessary.
Yellow articles should not be blued, since they turn greenish. Over-bluing is easily removed from fabrics by treatment with acetic acid or rinsing the fabric in plain water for 2 to 3 times more. Sunlight is the best natural bleach.
STIFFENING AGENTS USED IN LAUNDRY WORK
A certain amount of stiffness in the washed clothes gives them a smooth glossy surface, which is resistant to dirt and dust. The stiffness, however, must not impair the pliability of the garment too much.
It is a well-known stiffening agent used in laundry work for cotton and linen fabrics. It is a carbohydrate and stored by the plants in roots, seeds and tubers (e.g. potatoes sweet potatoes and arrowroot etc.) to foster growth during the coming season.
Types of starch
Starch grains obtained from different plants, vary in size and shape and are easily recognized under the microscope.
1. Rice starch:
These starch grains are the smallest and makes a viscous solution, which is suitable for stiffening the fabrics. It gives sufficient stiffness with pliability. This starch is suitable for cold water starching, as the size of the grains is small enough to effect an easy penetration into the fabric.
2. Wheat and Maize starch:
The grains are of intermediate size. They give a strong viscous solution leaving the fabric very stiff. Wheat starch is very expensive but maize is cheap and may be used after blending with other starches.
3. Potato starch:
These starch grains are very big hence not suitable for laundry purpose.
4. Tapioca starch:
Tapioca is obtained from the roots of cassava plant. The roots are dried, sliced and crushed to a pulp with water. The mash is washed with several changes of water and then evaporated to dryness. This starch is in the form of irregularly shaped lumps produced by breaking the solid white mass, which is left after evaporating.
5. Coloured starches:
Some starches are tinted to give shades of cream, green and blue.
6. Commercial starches:
Various brands of commercial starches are available in the market. These are usually prepared by blending two or three different kinds of starches.