I had a wonderful private workshop with Kate Crossley last week http://www.katecrossley.com/ She taught me how to do acid etching. We started with a piece of cotton fabric that she had already machine embroidered and corded with polyester threads (which are not affected by the acid).
We then layered the embroidered fabric with lots of other fabrics, placing the strips perpendicular to the embroidery on the base. We used cellulose, protein and synthetic fibers. When satisfied with the placement, we pinned the layers together.
We then machine embroidered the pieces so the stitches were perpendicular to the first set of stitches using polyester thread.
We applied a product called Fiber Etch at random on the layered, embroidered fabric, rubbing the gel into the fibers with the nozzle. Fiber Etch contains sodium bisulphate which eats away cellulose fibers (cotton, linen, rayon), but does not affect synthetics or protein fibers such as silk and wool; this is why polyester thread was used for the stitching.
We dried the Fiber Etch gel with a hair dryer (windows open, wearing a mask!), then the piece was placed between two layers of baking parchment and pressed thoroughly, with the iron on a wool setting. The etched areas turned a coffee brown color. We rinsed the fabric under running water, scraping away the etched fibers.
This is the result. It looks almost ethereal to me. Acid etching is a really fun technique with unpredictable results. Take a look at Kate's acid etched pieces on her website. She is available to teach workshops on this technique to groups.