Washing Instructions for Yarns Dyed With Weak Acid Dyes

Questions come up from time to time on how to take care of hand knits, especially because of all of the time and care that went into creating them; not to mention the often high cost in materials!  Let's face it, hand dyed yarn is expensive!!  That's why, when items are washed incorrectly and large amounts of dye releases (bleeding) from the yarn, it is heart breaking for everyone.  It's devastating for the knitter because loose dye often resettles on other areas of the knitted item causing changes in colors, usually not for the better.  But it's equally devastating for the dyer since knitters often assume that the bleeding was caused by poor quality dyeing.  Reputation is everything to a small artisan business and talk of poor dye quality can often mean the closure of these one-person ventures, even when the dyer isn't at fault.

The most common dyes, and the ones we use, belong to a class of dyes called weak acid dyes and may be purchased from numerous online and brick & mortar stores in a myriad of colors.  These little pots of dye powders are actually salts which dissolve in water, producing a negatively charged dye molecule.  Negative charges are attracted to positive charges and this forms the basis of the dye binding reaction.  Since the dye molecule is negatively charged, it binds to  positively charged sites within other molecules such as wool protein fibers.

Wool and silk are protein molecules. The number of positively charged sites within the protein is dependent on the pH of the solution.  At a pH of approximately 4.5, there are nearly equal numbers of positively charged sites and negatively charged sites on a wool molecule.  This is known as the isoelectric point.  Below the pH corresponding to the isoelectric point, there are more positively charged sites than negatively charged sites and above the isoelectric point there are more negatively charged sites.

With me so far?  Here's the take home message:

By changing the pH of the water in which you soak or wash the wool, you are changing the numbers of positively charged and negatively charged dye binding sites!!

When you wish to bind dye to the protein, you need to make sure the protein is strongly positively charged, or about 2 pH units below the isoelectric point.  This is the reason why an acid source such as vinegar or citric acid is added to the dye bath.  Vinegar (or citric acid) lowers the pH of the dye bath and creates numerous positively charged sites within the wool protein to which the negatively charged dye molecule can bind.

This is, hopefully, the situation with the indie-dyed yarn that you purchase.  It is straight-forward for the dyer to measure the amount of dye left in the dye bath or in the wash water using a visible spectrophotometer.  This allows the dyer to ensure all dye is bound to the yarn that the customer purchases. 

The same principles of pH that govern the binding of dye also control whether the dye remains bound during washing of the finished item.  Taking care that the pH is around pH 3 by adding vinegar (about 2 tablespoons for every gallon of wash water) ensures the dye remains bound.  The difference in dye fastness as a function of pH is profound as the following experiment demonstrates:

The dye colors that bind weakest are those belonging to the blue family, including turquoise and violets.  At very high concentrations, they become difficult to keep bound and the knitter must be especially careful to keep the pH low for all solutions that come into contact with intensely blue/turquoise/violet colored yarn.  Since this is the worst case scenario, I used 20 yard mini-skeins, each comprised of 5 yards each of turquoise, bright blue and 2 different violets.  Each mini skein contained the same amount of each of these 4 colors and each weighed the same amount.  I wound off 5 mini skeins:

Next, I filled jam jars with 150 mL of water at various pH values:  Left to right are approximately pH 2, 4, 6, 8, and 10.5.  They don't look any different from one another before the yarn is added, but they are VERY different on a molecular level!!  All jars of water were allowed to sit for one hour to come to the same temperature.


I added one mini skein per jar and submerged them for 15 minutes at room temperature:


Then I removed the mini skeins squeezed them out back into their respective jars, and examined how much dye came off into the water:



Look how little dye came off in the water at pH 2 and pH 4 (counting from the left, jars 1 and 2, respectively).  These pH values are below the isoelectric point for wool so the wool remains positively charged, attracting and holding the negatively charged dye ion tightly. 

Most municipal water supplies deliver water from the tap at close to neutral pH or pH 7, which is significantly higher than the isoelectric point of wool (ca. pH 4.5).  Jars 3 and 4 (counting from the left) contain the wash water at pH 6 and pH 8, respectively.  Municipal water pH is usually somewhere between these 2 values.  Look at how much dye is released by washing a finished item in tap water without adjusting the pH!!  This is despite the fact that all the dye was  fully bound to the yarn!  The dye bleed becomes progressively worse at higher pH values.

The dye release is strong enough at pH values above the isoelectric point and the that any pale colors that were used in making the garment will be targets for the dye resettling back onto the item, altering the original colors dramatically.  Intense reds or blues washed at neutral pH are notorious for settling on white turning them pink or turquoise, respectively.  However, addition of about 2 tablespoons of white vinegar to each gallon of wash water and rinse water prevents the dye from coming off and resettling, keeping those whites pristine.  

Finally, be very careful using any detergents, including wool wash.  Many of these products are buffered to keep the solution at neutral (and sometimes higher) pH, where the detergent works best.  Often, even with addition of vinegar these detergent solutions can pull dye off of yarn dyed with weak acid dyes.  If you must use a detergent, it is recommended that you test wash a swatch first to avoid unexpected (and unpleasant) surprises.  With a little extra care to keep the pH below the isoelectric point by addition of 2 tablespoons of white vinegar (per gallon of water) to the wash and rinse solutions, your projects will keep their vibrancy and beautiful colors!