Wednesday, 4 May 2011

Chuna Patri Printing Process of Bagru

It is amazing to know the variety of printing processes used in traditional printing in India. One of them is Chuna Patri process used in Bagru.

In this process a solution is prepared using Patri( Aniline Hydrochloride), Potassium Chlorate and Copper sulphate. 

Potassium Chlorate
Aniline Hydrochloride( Patri)
Copper Sulphate

Fabric is dipped into it and while it is wet, it is printed with lime + Gum. Lime acts as a discharge and doesn't allow the black color to develop on those areas. 

Fabric Printed with Lime
It is then dried in the sun. Every part of the fabric which is not printed first turns green and then jet black upon exposure to sun.  

Fabric Turning Green on Exposure to Sun
Fabric Turning Black Gradually
The problem is that the Patri and Potash makes the fabric weaken so post washing is very important.  To get a grey, black color is added with lime. It gives some of the best Blacks and white you can ever find. 

The Dye class is aniline black, used by Century Textiles long ago for dyeing their umbrellas. Patri is Aniline Hydrochloride, also known as aniline salt. It is a product that is made by aniline oil and HCl. This is in the form of white flake crystals and used for making black color in textile printing. 

It works on the principle that when a salt of aniline in solution is exposed to the action of oxidizers, it yields a black dye of such depth that other dyes look grey or green in comparison. In this case the aniline salt dissolves in water to give a acidic solution with a pH of around 5. 

Cotton  is dipped in the solution containing aniline salt, potassium chlorate, which acts as an oxidizer and copper salt, which acts as an oxygen carrier. The dyed material them becomes greener due to formation of Emeraldine and then turns jet black. 

When a wet solution is then printed with lime, the Ca(OH)2 will neutralize the selected areas as well as bleach them by forming calcium chloride, which gives such a brilliant white color. 

Faults during Patri Printing

1. Unevenness: This is mainly due to uneven impregnation. It also occurs when the fabric is allowed to come in contact with water before it is turned black. Even wet hands will lead to unevenness. 

2. Greening: This is due to insufficient oxidation. Often this is observed after some storage. 

3. Rubbing: This is due to the improper dyeing conditions and chemicals. 

4. Bronziness: This is due to over oxidation.

5. Tendering: During the formation of black a considerable amount of free Hydrochloric Acid may be formed, which upon acting on the cellulose at the elevated temperature is likely to attack cotton and produce tendering by formation of hydrocellulose. It can also happen if the fabric is aged ( exposed to air) for a prolonged period before washing. It is suggested that Ammonium Chloride should be added in the liquor to prevent this tendering. 

Further Studies

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