Monday, 4 March 2013

Dye Identification

I was looking for simple methods to identify class of dyes on fabric. Broadly I work with cotton, linen, silk and union fabrics of silk and viscose. The dyes that I am looking for to identify are direct, reactive, acid ( on silk), sulphur, vat, aniline black and napthols.

Here are some methods to test these dyes. I got the idea from this document. However I have not tested them so far.

1. Direct Dyes

                   This test of direct dye holds valid for both cotton and silks.

A 100-300 mg portion of the dyed sample is placed in a 35 ml test tube, 5-10 ml of water and 0.5 to 1 ml of conc. ammonia are added and the mixture is boiled in order to bleed off a sufficient amount of dye for redyeing a piece of white cotton cloth.

When a sufficient amount of dye has bled from the sample, the sample is removed, a piece of white cotton cloth weighing 10-30 mg is placed in the test tube and 5-30 mg of common salt is added. After boiling gently for 40-80 seconds and cooling to room temperature, the cotton is removed, rinsed and examined.

Redyeing on cotton in an ammonical solution in the presence of salt to shade and strength comparable to the shade and strength of the original sample is a direct evidence of direct dye.

2. Acid Dyes

                     For wool and Silk

A 100-300 mg portion of the dyed sample is placed in a 35 ml test tube, 5-10 ml of water and 0.5 to 1 ml of conc. ammonia are added and the mixture is boiled in order to bleed off a sufficient amount of dye for redyeing a piece of white cotton cloth.

The above part is same as acid dyes. However, if the sample in the direct dyes bled but left the test cotton white or only slightly stained, the colored extract is neutralized with 1 ml of 10% sulphuric acid solution and a few drops of acid are added in excess. A 20-30 mg of wool is added and the mixture is boiled for 1 to 2 min. The wool is rinsed and examined.

Redyeing of wool from an acid bath indicates the presence of acid dyes provided the presence of direct dyes is not shown.

3. Sulphur Dyes

                   For Cotton

A 100-300 mg of dyed sample is placed in a 35 ml test tube and to it are added 2 to 3 ml of water, 1 to 2 ml of 10% sodium carbonate solution and 200-400 mg of sodium sulfide chips.

The mixture is raised to a boil and boiled for 1 to 2 minutes. The sample is removed and to the test tube are added 25-30 mg of white cotton and 10-20 mg of common salt. After boiling for 1 to 2 minutes, the cotton sample is removed and placed on filter paper and allowed to reoxidize.

Under these conditions, sulfur dyes redye cotton in a shade which differs from the original only in strength. A few easily reducable vat dyes will color the white cotton but in a shade markedly different from the original dyeing.

Vat Dyes

    For Both Cotton and Protein ( For protein also follow the portion in Red font)

Vat dyes are to be tested after the sulphur dye test has come negative.

A 100-300 mg dyed sample is placed in a 35 ml test tube to which are added 2 to 3 ml of water and 0.5 to 1 ml of 10% caustic soda solution (The mixture is boiled till all the fiber is dissolved). After being brought to boil a 10-20 mg portion of sodium hydrosulphite  is added and boiling continued for another 0.5 to 1 minute. The sample is removed( Not Needed) and 25-50 mg of white cotton cloth and 10-20 mg of salt is added. Boiling is continued for 40-80 seconds, followed by cooling to room temperature. The cotton is removed and placed on filter paper to oxidize (then in a bath containing sodium nitrite and acetic acid).

Redyeing of cotton to a shade differing only in strength from the original dyeing indicates the presence of vat colors.

Napthol and Insoluble Azo Dyes

The most characteristic property of this class is bleeding in pyridine. A 10-50 mg dyed sample is placed in a 10-15 ml test tube, 1-2 ml of pyridine added and the sample boiled. All napthol bleed to certain extent.

Reactive Dyes

Boil in water with a temperature more than 60 deg in Sodium hydrosulphite and caustic soda as in vat dyes. First the color will come out and then that color will decolorize.

Saturday, 2 March 2013

Indian Ethnic Fabric Buying- Evaluating Vendors -2

Traditional techniques used by a particular vendor also make him important in terms of buying. Traditional techniques make the vendor unique among others and make him valuable in terms of planning as the techniques demand more lead time and quality checking. It assumes more importance if the vendor belongs to the same region which is the origin of these techniques. For example vendors supplying Dabu, Bagru, Ajrak, Bandhni, Lehariya and Patri from Rajasthan are rated higher as they are using the traditional techniques in printing the fabrics. For similar reason block printing vendors are rated higher than screen printing vendors and vendors using handloom are rated higher than those using powerloom.
Exclusivity of the vendor for a particular company makes him valuable. As the vendor is not supplying to any other company, the possibility of desgins and language leaking to ther other compaies is obviated. Also obviated is the fluctuations in supply as the whole capacity of the vendor is planned when ordering fabrics or garments. 
Percentage of quality rejections is one of the most important factors in evaluating the vendors. More quality rejections will not only throw a question in the vendors' ability to supply a particular quality of fabric, it will also throw question on the viablity of bulk produciton of that traditional techniques.An example of this is very pertinent here. A retail organisation traditionally does Milspun x Khadi yarn in one of its most selling garments. When they tried to do it in Khadi x Khadi yarn, the number of quality issues become so high that they have to abondan that design using fabric.
Price is a very important factor in evaluating a vendor. A vendor traditionally supplying goods at reasonable price are rated higher. However, in traditional textiles, low prices have their own social and envinornmental costs. In handloom weaving for example the production is distributed by traders to the weavers at a pittance in order to keep down the prices. Similarly the traditional prints are sold at lower costs because the printers still have not heard of affluent treatment plants and the affluents are simply  added in the rivers or canals. But high prices are also no guarantee of the quality of the merchandise. I know a printer who does excellent block printing in terms of design. Every year the prices are raised in order to save the block printing and the printers with the result that his prices have become inordinately high.
<-- To be continued-->
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