Scour (clean) the cotton or linen fibers well. Fill a dye pot with enough warm water for the fibers to move around freely. Add 1 gram of soda ash (sodium carbonate), plus 1 gram for each additional pound of fiber. Add the fibers and raise the heat to a very gentle simmer. Simmer the fibers for one hour, stirring occasionally. After an hour, remove from the heat. When cool enough to handle safely, rinse well with warm water until the water runs clear.

Prepare the fibers by treating them with a tannin like Sumac Extract, Oak Gall Extract, Oak Gall Extract Light, Myrobalan Extract, Pomegranate Extract, or Chestnut Extract. Dissolve 10-12% weight-of-fabric (WOF) of the tannin extract into a dye pot with just enough warm water to cover the fibers [see Note 1]. Add the cotton or linen and then bring the tannin bath just to a boil. Immediately remove the pot from the heat, cover it, and let the fibers steep overnight. Stir them occasionally so that they absorb the tannin evenly. Before going to bed, make sure that there are no bubbles trapped underneath the fibers and that the fibers are fully submerged.

When ready to proceed the next day, lift the fibers out of the tannin bath and gently squeeze excess liquid out (just enough so that they are not dripping). Do not rinse them. The tannin bath may be reused so long as it is not cloudy, although it may need to be topped off with more of the tannin extract.

Fill a dye pot or plastic bucket with enough soft or distilled water for the fibers to move around freely. The water should be room temperature or very slightly lukewarm, around 80-90°F (27-32°C).

Weigh out 10% weight-of-fabric (WOF) of Aluminum Formate Pro. Add to the mordant bath and mix well until fully dissolved. The mordant bath will turn a milky white color.

Gently add your tannin-treated cotton or linen fibers to the mordant bath. Wearing rubber gloves, work the fibers in the liquid to make sure they are evenly saturated. Make sure they are completely submerged and no bubbles are trapped underneath.

Cover the pot or bucket and let the fibers steep for 2 hours. Stir the fibers a few times while they are steeping to make sure they mordant evenly. After 2 hours, the cotton or linen will be thoroughly mordanted with aluminum (although the fibers may not look any different at this stage, especially if you are using a clear tannin).

[Optional] This step is optional but recommended. Prepare a chalk fixing bath: fill a bucket or dye pot with hot tap water (around 110°F/43°C). Measure out 2 grams of chalk (calcium carbonate) per liter of water. Add the chalk and mix well until dispersed. Wearing gloves, remove the cotton or linen from the mordant bath, gently squeeze out, then place it in the chalk fixing bath. Let the fibers steep in the fixing bath for 5 minutes [see NOTE 2]. After five minutes most of the excess mordant will be precipitated out of the fibers and the fibers can be removed and rinsed.

Rinse the fibers. Wearing gloves, remove the cotton or linen from the mordant bath or fixing bath (if used) and rinse well until the water runs clear. It is important to rinse out excess mordant and tannin at this stage, because otherwise it will cause trouble later on in the dye bath (some of the excess mordant may bleed out into the dye bath and waste dye, and some of it may fix superficially to the surface of the fibers and make them tedious to rinse).

When the cotton or linen has been thoroughly rinsed, proceed directly to dyeing. The swatches above show cotton tshirts mordanted and dyed with Aluminum Formate Pro, using different combinations of tannins and dyes [see Note 3].

NOTE 1. The amount of aluminum that the cotton or linen will absorb during the mordanting step will depend entirely on how much tannin is soaked into the fibers. A more concentrated tannin bath will result in more mordant absorption, and potentially darker colors after dyeing. If you need especially dark colors, do not calculate the amount of tannin based on the weight of the fibers (e.g., 10% WOF). Instead, make a concentrated tannin bath using 5 or 10 grams of tannin extract per liter of water.

NOTE 2. The purpose of this fixing bath is to precipitate out any excess aluminum mordant that is not firmly attached to the fibers. There will always be some portion of the aluminum that is not bound by the tannins, and it can be removed by precipitating it with a weak alkali like chalk. Otherwise, it may cause issues later on. Superficially attached mordant is liable to rub off the surface of the fibers, which can result in bleeding and crocking after dyeing is complete. It is much easier to remove this excess mordant with a quick fixing bath, rather than to try and rinse it out later. With some dyes (especially dyes with lots of tannins, like cutch or raw madder root) the fixing bath might not be necessary because the tannins in the dye itself can fix the excess mordant. However, when working with highly purified dye extracts like Wild Madder Extract or Royal Logwood Extract, the fixing bath will help in preventing later bleeding and making the cotton or linen easier to rinse.

NOTE 3. Different tannins have different advantages. In general, oak gall tannins will fix the most mordant and result in the darkest, most opaque colors (especially on thin fabrics like tshirts). Sumac tannins are noted for producing very clear, bright shades, but the color will not be quite as dark and opaque. Chestnut and myrobalan will add a distinctive brownish note that can complement dyes like madder and cutch.