 |  | Dyeing of Leaves and Straws: A Handbook for Craft Instructors (NRI) | ||
 |  | Methods of applying dyes | ||
|  | Preparation of material | ||
| | Acid dyes | ||
| | Basic dyes | ||
| | Disperse dyes | ||
| | Preparing the dyebath | ||
| | Dyeing | ||
| | After-treatment | ||
| | Finishing | ||
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By using material that has already been cut and trimmed for weaving, dyeing costs will be kept to a minimum. Normally the material should be thoroughly wetted before dyeing. This can be done quickly by boiling it for 30 - 60 minutes in water containing a wetting agent, then leaving the material in the bath while it cools. However, when using disperse dyes it is not necessary to wet the material.
Choice of dyes
Most acid dyes cannot be used with leaves and straws. However, among a large number tested on big thatch a few were found to be suitable (see Table 1(b)). It was found that these same dyes (especially the 1:2 metal complex dyes) were, in general, also largely suitable for other palm leaves such as palmyra (Borassus flabellifer) and doum palm (Hyphaene spp.), pandanus leaves and vetiver grass. (Jippi-jappa was an exception in that it would take up very few acid dyes and it is better therefore to use other classes of dye on this material.) With an unknown material it is well worthwhile, therefore, to experiment first with those dyes which give good results with big thatch.
Because of the 'bleeding' of dye from the split edges of the straws, most acid dyes give rather poor results for water fastness. A substantial improvement is obtained if the freshly dyed material is soaked for a short time in very hot water to remove surplus dye. The leaves could be dyed before splitting, but this would waste dye as some parts of the leaves are discarded.
Dissolving the dyes
The dye powders are mixed to a smooth paste with water. The paste is then added to boiling water, using additional water to rinse the dye paste from its container. Boiling is continued for 2 - 3 minutes with stirring, to dissolve the dye. Each 10 9 of dye will need about 1 litre of water to dissolve it completely.
Acids to use
Acid must be added to the dyebath, in order to fix the dye. Acetic acid is suitable for use with the 1:2 metal complex dyes (e.g. Bayer 'Isolan' dyes) and, with these, 4 9 of the 30% strength acid is to be used with each 100 9 of material, regardless of the depth of colour. However, other types of dye may require a stronger acid and usually it is recommended that 3 - 5 9 of 85% strength formic acid is used for each 100 9 of material dyed. The ideal amount depends to some extent on the nature of the individual dyes, but the deeper shades usually require larger amounts of acid.
With straws, the acid can usually be put in the bath at the start of dyeing. However, if there is difficulty in distributing the dye evenly over the straw surface the acid should be added a portion at a time, throughout the dyeing period.
Choice of dyes
Almost all basic dyes are readily taken up by leaves and straws to give strong colours. They are therefore widely used for craft work since short dyeing times and small amounts of dye can often be used to produce cheap colours. However, the fastness properties of these cheap colours are poor and the basic dyes cannot be generally recommended for use on durable goods.
Selected basic dyes will give moderately good light fastness
with good water fastness when properly applied to iraca straw and jippi-jappa
(see Table 2(a)). The same dyes also produce good results on big thatch,
pandanus and vetiver grass. Therefore, when dyeing an unknown material for the
first time it is well worthwhile to experiment with the few basic dyes which
give the better fastness properties on other materials.
Only a few of the
many dyes investigated are suitable for blending to give a range of fast
colours. This range will be limited by the colours of the best dyes, but may be
extended by the use of selected disperse dyes in the blend. Basic and disperse
dyes should not be blended before the powders have been mixed with water.
Dissolving the dyes
The dye powder is first wetted with a little industrial spirit (methylated spirit) or acetic acid to prevent it from forming sticky tars with water. The dye is then mixed to a smooth paste with water, making certain that no lumps remain. Hot water, near the boil, is added with stirring until the dye is dissolved. At least 1 litre of water for every 10 9 of dye will be needed to make a solution.
Acetic acid is used in the dyebath to assist even dyeing. Between 2 9 and 5 9 of 30% strength acetic acid is sufficient for each 100 9 of straw dyed; the larger amounts of acid are needed with pale colours. The acid is added before dyeing commences and some of it is used initially to paste the dye. Vinegar (between 12 9 and 30 9 for each 100 9 of material) can be used instead of acetic acid.
Choice of dyes
Disperse dyes, including the reactive disperse (ICI 'Procinyl') dyes, are readily taken up by leaf and straw materials but few give colours which combine good light fastness with good water fastness. However, the dyes are very easy to use and the use of selected members of the class should be considered - especially for jippi-jappa, which does not take acid dyes very well.
Promising dyes can be selected from the large number evaluated on iraca straw and jippi-jappa (see Table 3(a)). A number of the same dyes have been evaluated on big thatch and pandanus with similar results. Although none have been evaluated on vetiver grass it is well worth experimenting with the more promising dyes on this, or any other material, which needs to be dyed.
Some disperse dyes build up to pale colours only (particularly Cl Disperse Red 60*) and care should be taken to ensure that excessive amounts of dye are not used in the bath. This will limit the range of fast shades that can be blended: for deep shades, especially those containing red, it may be better to use basic dyes or blends of basic and disperse dyes. Basic and disperse dyes should not be blended before they are mixed with water.
Dissolving the dyes
Disperse dyes do not dissolve in water, but contain auxiliary agents which suspend them in water as finely-divided particles. It is important that the correct amounts of water are used when initially dispersing the dye particles otherwise the dispersing agent in the dye may not be effective.
The dyes are available either as fine powders, grains, liquids or pastes. The method of dispersing the dye into water is different with each:
(i) Fine powders are sprinkled, with stirring, into water which is just too hot to touch (50°C) using 10 - 20 ml of water for each gram of dye. The mixture is then left to stand for 5 minutes with occasional stirring.
(ii) Grains are dispersed by pouring them onto moving, hot, but not boiling (50 - 80°C) water using 10 - 15 ml of water for each gram of dye. The mixture is then left to stand for at least 5 minutes before stirring. The mixture is ready for use when no grains remain in the liquid.
(iii) Pastes and liquids are stirred into warm water.
The use of a wetting and penetrating agent (a form of detergent) in the bath will help to obtain even dyeing. For straw dyeing, a 'non-ionic' type, such as Synperonic BD or Metapol HC, is recommended. Both of these non-ionic wetting agents are available from Durham Chemicals Distributors Ltd (see Appendix for address). Washing-up liquids are often of the 'anionic' types and are likely to interfere with the dyeing process. However, it may be worthwhile to experiment with various brands. The amount of detergent needed is of the order of 1 9 for each litre of the dyebath (1 teaspoonful equals approximately 5 g).
Normally, 20 - 30 litres of dye liquor are needed for each kilogram of straw. However, if the straw is bulky it may be necessary to increase the amount of liquor in order that the straw can be both submerged, and freely moved through the liquor. The pre-dissolved dye is added to the bath and made up to almost the required volume with cold water. With acid and basic dyes it is sometimes
convenient to prepare the dye solution directly in the dyebath, using one-quarter to one-third of the total volume of water that will be used for dyeing. The hot solution can then be diluted with the remaining cold water to reduce the temperature of the bath to about 50°C. This is about the correct temperature to start dyeing. Soft water (e.g. rain water) is best, but with acid and basic dyes it should not be alkaline. Alkaline waters must be neutralized with additional acetic acid before dyeing. The detergent and acids are added as required. The bath is made up to its full volume with water and stirred well to mix the ingredients.
The material is placed in the dyebath which is then slowly heated, with stirring, to the boil. (This warming up period is the most important part of the dyeing process since it is in this period that even, or uneven, colours are made. Usually, a period of about 30 minutes is sufficient, but longer periods can be used at the dyer's discretion, to ensure that the dye is deposited evenly over the material.)
Dyeing, at the boil, is continued for at least one hour. Further increases of colour intensity and depth of penetration can be obtained for most dyes by increasing this time to 2 hours or more. The exact time needed (for each dyeing) can only be determined by experience. Acid dyes usually require more time at the boil than basic or disperse dyes.
Efficient circulation of liquor round the straws is difficult to obtain. Repeated steeping of the material is recommended. Alternatively, special equipment could be designed in which the straight laid material is tumbled in a horizontally placed drum.
After dyeing at the boil, the material can be removed from the liquor. However, if left in the cooling liquor the material will absorb more dye. The dyer must decide whether or not to leave the material in the cooling liquor.
The dyed material is removed from the dyebath and rinsed in cold, preferably, running water. This removes adhering dye liquor and thus prevents the formation of loose dye powder on the surface of the material during drying. Finally, the material is spread in the shade to dry.
Dyed straws tend to become brittle on drying. Some will recover their suppleness if they are first re-wetted in cold water for about one hour, then redried. Straws which do not have their suppleness restored by this treatment will often benefit from immersion in a 10 per cent aqueous solution of glycerine, followed by drying. If the dried material looks wet, the concentration of glycerine should be reduced. Unfortunately there is often a considerable loss of dye during this treatment. Immersion for about 2 hours in acrylic emulsions (e.g. Vinacril 4000) diluted with water to a resin solid content of 2 per cent gives a considerable improvement in handle particularly with big thatch and silver thatch. These treatments always produce a shade change due to the swelling of the material.
Some leaf materials become brittle with age (Jayaraj and Sivaramalingham, 1967). This is attributed to fungal and bacterial attack. Treatment for 20 minutes in a 1 per cent aqueous solution of sodium pentachlorophenate, also containing 1 9 of wetting agent for each litre, followed by treatment in a 5 per cent aqueous zinc sulphate solution will overcome this problem.
Safety note: With sodium pentachlorophenate there is serious risk of poisoning by inhalation, swallowing or skin contact. Wear gloves and a facemask when handling the chemical. Spillages should be mixed with sand and buried in a safe open place and the site of the spillage should be washed thoroughly with water and soap or detergent. Straws are usually easier to weave if they are dampened slightly before weaving. Some weavers store their material in damp newspaper for a few hours before using it.
Finishing agents can be used to increase the stiffness or the flexibility of straw goods and to enhance their lustre and water resistance. Some agents will improve several properties at the same time.
Water resistance is a particularly important consideration since straw goods readily lose their shape in damp or humid conditions.
Frohlich 11963) gave examples of commonly used agents for stiffening. These included glue, gelatine, mucilage (tragacanth), starch and dextrin, which are all applied in similar ways: for gelatine, the work is dipped in a hot solution containing 30 - 40 9 of gelatine for each litre then dried at moderate temperatures. However, these agents are only partially successful, and are not water resistant. Appretan (plastic dispersions from Hoechst AG based on polyvinyl acetate), and shellac solutions, particularly in alcohol, are more suitable. Collodion solutions (nitrocellulose lacquers) and natural resins are also examples of stiffening agents. However, only cellulose lacquers which are flexible should be used and manufacturers should be consulted about the formulation of a special 'dope' for the purpose (Martin, 1938). Martin also records that a synthetic product (probably a formaldehyde resin) gave many desirable properties and was superseding gelatine as a stiffening agent for sisal hats.
Flexibility can be improved by treatment of the straw in a solution of glycerine (glycerol) or a soluble oil after dyeing. For some straws, as little as 10 ml of glycerine in each litre of solution will suffice when treatment is for 15 - 20 minutes in a standing bath at 40°C (Martin, 1938): sponging with a solution containing 50 ml of a soluble oil and 10 ml of glycerine in each litre is given as an alternative treatment for straw which is to be pressed under high pressure. Waxes are also included with other finishes to impart additional water resistance and flexibility (Frohlich, 1963). These finishes just described, particularly cellulose lacquers, usually give the desired lustre.
At the Tropical Development and Research Institute (TDRI), preliminary trials have been carried out on big thatch using a wax from Catomance Ltd, and some synthetic resins from Vinyl Products Ltd. The wax, Mystolene SP30, was applied by dipping the hats in a solution containing 50 g of wax in each litre of a non-polar solvent (e.g. white spirit) then allowing them to dry. The resultant hats, which were slightly yellowed by the wax, could be filled with water then shaken dry, without loss of shape. Vinalak 5920 (Vinyl Products Ltd) similarly applied" using a solution containing 5 per cent solids gave a lustrous, colourless flexible finish that was also water resistant.
A wide variety of resins either in organic solvents or emulsified with water is available from Vinyl Products Ltd and many of these could be useful to rural workers for imparting special qualities to their straw goods. Trials at TDRI showed that thin coatings to provide lustre and water resistance were best obtained using the resins in organic solvent, since thin coatings from emulsions were permeable to water (thicker coatings gave the straw the appearance of plastic). The Vinalak 5920 was the more flexible of the resins in organic solvent used in the trials. However, resins and finishes can be modified to suit particular applications and rural workers are advised to consult manufacturers about their specific needs.
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