I promised in the last post that we’d spend more time talking about various chunks of water chemistry, a very broad overview, as it relates to dye chemistry. In specific, madder dye baths. Well here we are, another post about water. Today we’re going to have a quick look at Total Dissolved Solids, or TDS.
I measured the TDS values out of all of my samples with a basic meter (acquired off of Amazon), and gave it a rinse with distilled water between each sample. It was the meter that gave me the least grief throughout the entire experience, so gold star to my little TDS meter.
When we’re discussing things that are dissolved in our water, that is exactly what the TDS measure is. Total Dissolved Solids, which describes the both the inorganic salts and the small amounts of potential organic material present in the water. This could be a while assortment of things, but are usually calcium, magnesium, sodium cations and carbonate, chloride sulfate anions, amongst others. While TDS is often assumed to be a measure of quality, it is less discriminatory than that. It only measures that something is there, not what that something is, nor if that something is harmful, or unpalatable. The WHO has very general guidelines on palatability of drinking water with less than 300 mg/litre being considered excellent, good, between 300 and 600 mg/litre; fair, between 600 and 900 mg/litre; poor, between 900 and 1200 mg/litre; and levels greater than 1200 mg/litre being considered unacceptable.1 In our water survey TDS was measured in ppm (parts per million), which is approximately equivalent to mg/litre. The lowest TDS measured (outside of controls) was sample 37 at 27ppm, which falls in the excellent category and the highest was sample 26 at 559 ppm, which falls in the good category. A copy of my water survey results can be found linked in my google drive.
Often, it is assumed that TDS is a measure of hardness, but TDS is too generalized a measure to mean only that. While it includes the minerals that are part of what is referred to as ‘hardness’, primarily calcium and magnesium, that is not all that it includes. The TDS values of the iron samples are a prime example of that (samples 63, 65-68). Those samples are distilled water with the addition of ferrous sulphate, no minerals associated with hard water present and they not only have TDS values, but it is proportionate with the amount of ferrous sulphate added. In our water samples, it is overly simplistic to assume that TDS refers to hardness, but in absence of good detection of other minerals, it is an assumption I am generally willing to make, with caveats. Certainly I do not assume TDS references hardness when the composition of the solution is known (ie the ferrous sulphate solutions). Water in Ontario is extremely variable in hardness, although we do have some of the hardest water in the country, but that is not consistent.2 Madder, as a dye, appreciates hard water, it usually promotes a deeper red colour, although interestingly, we got an exceptional colour from distilled water (sample 60), so clearly no one told my madder bath that day.
Modern dyers, when looking to augment the calcium in their water, tend to add chalk (calcium carbonate) directly, or a crushed up tums tablet or two (which is calcium carbonate, but with handy fruit flavours!), so if you aren’t super happy with the colour you get from your madder pots, give that a try. Calcium carbonate is very mildly basic, so it doesn’t affect the pH of the dye bath to any great extent.
Next up, we’ll have a look at iron, which can be contained in the TDS measure, but is interesting enough to talk about all on it’s own.