Immersion Dyeing in a Vat

There are two basic types of immersion dyeing: vat or garment dyeing and the soda ash soak method. Vat dyeing is most useful if you are dyeing large pieces of fabric or articles of clothing and you wish to produce an even color. You can use a bucket, bathtub or large pail to hand dye up to 1 pound (about 3-6 square yards) of cotton or other natural fiber fabrics or about 3 T-shirts. For larger amounts up to 8 pounds of fabric, you can use your washing machine by setting it on manual or by watching it carefully so that the water does not drain too soon.

The basic equipment and supplies for using fiber-reactive dyes are minimal and inexpensive. You will need:

dust masks or respirator.
funnel if you are using a gallon jug.
gallon container - this can be a plastic milk jug, pitcher or bucket that you can pour from or dip from with a measuring cup.
measuring cups and spoons.
containers for mixing the dye stock - can be clear plstic cups or recycled glass jars. Clear ones make it easier to check the bottom to make sure that all the dye powder has been dissolved.
mixing sticks - these may be plastic or wood chop sticks, disposable plastic knives, tongue depressors or craft sticks. Plastic may be cleaned and re-used, wood can only be used once because it absorbs the dye and can distort the color of another solution.
paper towels or rags to clean up spills.
heavy plastic freezer bags or glass jars - a gallon bag holds 1/2 yard of fabric; a 1/2 pint jar holds a fat quarter.
plastic dishpans - one to hold the bags of dye and fabric (if you use this method) and one for rinsing utensils between colors.
plastic drop cloth or newspapers to protect work surfaces.
rubber gloves.
non-iodized salt to help activate the dye.
soda ash (sodium carbonate), also known as washing soda, to help the dye bond with the fibers. Soda ash can irritate sensitive skin and can make dust so it is best to wear rubber gloves and a mask while you are measuring it. If you get washing soda from the laundry isle at the supermarket, make sure it is pure and does not have the little blue "booster" crystals in it as they can affect the chemical solution.
water softener - if you have hard water and want a nice even color, you may want to add water softener to your water. I don't bother because I am looking for the uneven color and texture.
Synthrapol or other mild detergent as a pre-wash to remove invisible lubricants, dirt, oil and other impurities that may cause uneven dyeing and as an after-wash to keep the excess dye particles in suspension so they are not redeposited to stain other areas of the fabric.
large tub if you are going to do hand dyeing in a vat.

Here is the basic method to vat dye 1 pound of fabric:

Prewash the fabric using Synthrapol to remove any dirt and grease and a strong enzyme detergent to remove and sizing or starch.
Dissolve 3 cups of plain salt in 3 gallons of lukewarm water (about 105°F). If you have hard water and don't want uneven dye, you should use a water softener in the water. If you are using Calsolene oil (a surfactant which breaks the surface tension of the water so that it can wet the fabric better, resulting in a more even color), add 2 teaspoons after the salt is dissolved.
Dissolve the dye (about 1 tablespoon per pound of fabric) in a cup of warm water, add to the water and stir well to dissolve the dye evenly. The 1 tablespoon per pound of fabric is a general average amount to produce a medium hue but check the dyes or a chart because some colors require more of the dye powder (up to 4x in some cases) to produce the same intensity.
Add the fabric and stir constantly (well, OK, at least every 3 or 4 minutes) for 20 minutes.
Dissolve 1/3 cup of soda ash in some hot water and add it slowly, while stirring, to the dye bath over a period of 15 minutes. Avoid pouring it directly on the fabric. Let sit for 30 minutes (for light colors) to 1 hour (for deep colors), stirring frequently.
Remove the fabric from the dye bath and rinse to remove excess dye.
Wash the fabric in hot water with Synthrapol to remove the rest of the dye.

References and further reading:

Hand-Dyed Fabric Made Easy by Adriene Buffington
Dye Painting by Ann Johnston
Thread Magic - The Enchanted World of Ellen Anne Eddy

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