- What is the correct method of mixing colours?
- Is there a standard process of thinning paste that can be used for block prints?
- How do you make binders suitable for block printing?
- A paste left open to air, or kept over long time tends to dry up and forms a dry layer. How can this drying/ formation of crusts be prevented?
- For hand printing units that use pigment colours, it is observed that a halo is formed around the shade when using water binder. A halo is a thin ring of a lighter tone (of the colour printed) that forms around the printed area
- For hand printing units that use pigment colours, it is observed that from time to time, Khari colour (Titanium di-oxide, TiO2) colour turns green on drying
- Metallic colours such as gold and silver have poor colour fastness, and tend to come off on washing or rubbing.
- Metallic colours such as gold and silver dry up very quickly and are therefore not transferred from block to fabric.
- In cases where very fine prints are required, say, for example, very thin lines, or fine design, use of water based binder does not yield desired result. Use of water based binder leads to spreading of the fine lines, therefore smudging the design.
- Is there an alternative for “Gum Arabica”?
- How to identify if the base fabric is processed correctly? A fabric that has not been treated properly can cause a multitude of problems, including, but not limited to:
- Silk becomes weak in areas where it has been printed on, and tears on stretching. What is the root cause? What are possible ways to prevent this?
- The printing paste catches fungus during the day
- The pH of the printing paste changes over the day with Indigosols
Mixing of colour is one the most important aspects to ensure quality of printing. The colour should be added at the end, once all the ingredients are added. Constant stirring of the paste must be ensured at that time.
A thinning paste needs to be added to dilute the colour paste so that it can be used fine prints.
A common composition is to use binder and water in a 50:50 ratio. Higher quantities of water will result in lower viscosity of the paste, which will lead to spreading of the print. Binder is a chemical which forms a thin layer on the fabric. The colour binds to this layer. High quantities of binder will result in harsh feel of fabric at the printed portions.
Binder is a chemical which forms a thin layer on the fabric. The colour binds to this layer. High quantities of binder will result in increased thickness of the layer, and will therefore give a rough feel to the fabric.
Binder supplier’s recommendation must be followed. Ususally 8-10% concentration i.e. 80-100 gms of binder per Kg. of print paste is suggested.
Any binder must be relatively soft and flexible in addition to good film forming capacity and stability. An ideal binder should be able to form the binding layer with minimal quantities of it being used.
Formation of crust is due to temperature and air quality.
Likely solution approaches: The pad should be continuously scraped and fresh paste to be added. An additional suggestion is to use 20 gms of urea per Kg of print paste which would act as a hygroscopic agent to prevent crust formation.
Likely causes:There are two likely causes for the halo being formed:
a. Some solubility of the pigment in water
b. Breaking up of the print paste
Likely solution approaches:
a. The pigment characteristics should be confirmed from the manufacturer, particularly the solubility in water
b. The storage of the paste should be taken proper care of. It must be made sure that the paste is not left out in the open, and that it is covered with a cloth when not in use.
Likely causes: Overheating is the likely cause for this problem. For pigment colours, it is essential to cure the fabric (pass it through dry heat) to fix the colour. While using a curing machine is the ideal way of doing this, most units use hot iron instead. Use of hot iron can result in lack of temperature control, and hence the change in colour.
Likely solution approaches: The heat sensitivity of the pigment should be checked, and the temperature of the iron should be maintained accordingly. If possible curing machine should be used instead of hot iron.
Likely causes: For good fastness, the colour should properly adhere to the binder. Insufficient adhesion of the colour to the binder will lead the colour to come off during resistance.
Likely solution approaches: The manufacturer should be consulted to make sure that the colour quality is appropriate, and if a suitable binder exists for such colours. An additional suggestion is that the binder and fixer concentration may be increased to 12-15% and 3-4% respectively.
Likely causes: High concentration of colour powder in the paste results in quick drying, therefore resulting in the problem
Likely solution approaches: This can be improved by adding hygroscopic agent like urea or glycerine to the colour paste. A hygroscopic agent has the tendency to retain moisture, and therefore does not allow the paste to dry easily. If this does not work, the colour manufacturer will need to be consulted for appropriate recipe.
Likely causes: Water solubility of the colour is the likely cause of this problem. Water spreads on the fabric, and if the colour is soluble in water, spreading of water will also take the colour along with it.
Likely solution approaches: Use of metallic blocks is one possible solution as they can be made with a finer precision. An additional suggestion is to increase viscosity of print paste by using higher concentration of thickener.
“Gum Arabica” is used more as an adhesive than as a thickening agent, and for this purpose, concentrations as high as 40% may be required
Likely solution approaches: Karaya gum (20%) can be used as a cheaper alternative. Crystal gum is a preswollen and purified material made from vegetable gums such as Karaya, and is readily soluble and more reproducible in properties than the original gum.
• Poor fixation of colour
• The print having improper depth
• Colour variation
There are three easy ways to check the fabric quality:
a. Whiteness: This will affect the contrast between the base fabric, and the print. A poor whiteness index will also result in getting improper tone of the colour on the fabric.
b. Absorbency: A fabric which has poor absorbency will not pick up the colour, hence leading to colour matching problems. Poor absorbency will also lead to the colour not giving proper depth.
c. Iodine test for presence of starch: Presence of starch on fabric before printing would act as a barrier to prevent the colour to penetrate inside the fabric, leading to low fastness properties.
The tests for these have been explained in the chapter on test procedures.
Likely causes: This can happen due to a variety of causes, including:
a. High alkali content
b. High steaming time
c. The steam may be saturated
Likely solution approaches: The steaming time should be between 30-45 minutes, and it must be made sure that only dry steam is used for heating. The alkali used should be very mild (Sodium bicarbonate can be used. Soda ash is a strong alkali and may cause the tearing of silk.)
Sodium benzoate can be added as a stabilizing agent to prevent formation of fungus.
Likely causes: Due to photosensitivity of Indigosol dyes some oxidation occurs and it may shift pH to acidic which may be the reason for colour change.
Likely solution approaches: A small amount of soda ash may be added to the paste to neutralize and stabilize the paste.