Dyeing Fabric: Mordants, Bleach, and Colorfastness
Have you ever noticed that sometimes certain shirts and jeans, especially when they are navy blue or black, tend to look duller after they’ve been laundered a few times?
The term colorfastness describes how well a fabric is able to hold dye through wash and wear. Colorfastness is dependent a few factors, including what the fabric is made of, what the dye is made of, and if any other agent is added during the dyeing process to help the color bond with the material.
Problem: What makes a dyed fabric most colorfast?
- 2 pieces white silk fabric
- 2 pieces white cotton fabric
- 2 pieces white polyester fabric
- Liquid fabric dye
- 3 old pots of equal size (not cooking pots)
- Cream of tartar
- Kitchen scale
- Permanent marker
- Find two pieces of silk, two pieces of white cotton, and two pieces of white polyester that are each about half a foot by half a foot square.
- Using a permanent marker, label the silk “S,” the cotton “C,” and the polyester “P.”
- Weigh one of each piece of fabric and note the total weight of all of the pieces of fabric. Multiply the weight of the fabric by 10% to get the weight of alum you need. Multiply the weight by 5%to get the weight of cream of tartar you need.
- Fill one pot with water and heat it. Make your mordant by mixing alum and cream of tartar with some hot water, and then add it to the pot.
- Add a piece of silk, a piece of polyester, and a piece of cotton to the pot, making sure that the fabric is completely covered by the water.
- Let the fabric simmer for one hour in the mordant solution. After that hour, remove the fabric, squeeze out the water, and wash out the pot so that no alum or cream of tartar remains.
- Fill up the pot with water again and put the same pieces of fabric back into the pot. Add 10 drops of liquid dye. Stir the fabric for five minutes. Remove it from the pot, and let it dry.
- Fill the second pot with water and heat it. Add a piece of silk, a piece of polyester, and a piece of cotton to the pot, making sure that the fabric is completely covered by the water. Let it simmer for one hour, then remove the pieces of fabric, squeeze out the water, and wash out the pot. Fill up the pot again and put the same pieces of fabric into the pot. Add 10 drops of liquid dye. Stir the fabric for five minutes, remove it from the pot, and let them dry.
- What pieces of fabric are the brightest? Which ones are the least bright? Why?
- Place the six pieces of cloth onto some old newspaper, and use an eyedropper to place drops of bleach onto the cloth. What happens?
The fabric that was boiled in the alum and cream of tartar has the strongest color. Cotton tends to be the most predictable of the three fabrics and it will get the most consistent color. The polyester will not absorb dye well.
A mordant is a substance used to help dyes connect with material at a chemical level. It forms a coordination complex, an atom or ion that is chemically bonded to a number of molecules or anions. In the case of a mordant, this binds the dye and the fabric together. Alum and cream of tartar form a mordant. When you compare cloth that was boiled without a mordant and then dyed to cloth that was boiled with a mordant, the cloth with the mordant will be more able to bond with the dye.
Of the three different fabrics, cotton is the most consistent and accepts dye the most easily. Synthetic fabrics tend to be difficult to dye and polyester will only accept dye if it is boiled in water at 160 to 200 degrees F.
When you added bleach to the fabric, you removed the color from the fabric. The cloth with the mordant has the best bonds between fabric and dye, but bleach works on all of the chemical bonds to break them down, removing the color from the fabric.
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