Orvus, an "equine soap," is the preferred method of washing needlework prior to finishing. It is a wetting agent (not a soap), and this is good because it just "makes water wetter" and doesn't leave residue on the piece. Residues attract soil. It's because your skin leaves oil on your work (even if you wash your hands often while stitching) that you must wash it before finishing.
Can you substitute a soap, such as Ivory Liquid?
Information from my chemist cyberfriend Kay Hahn, an environmetal chemist:
Sodium lauryl sulfate is a wetting agent. Chemically, it is not a soap.
What Orvus does is make water wetter. Whereas regular water drops bead up on a plate, Orvus-water drops lie flat in a very thin sheet. This means an Orvus-water mixture gets into the fibers of the fabric easily and float crud out, where straight water just makes beads on the fabric and doesn't do much in the way of cleaning.
In mixtures with cement, sand, and water, the Orvus makes the water wet the cement and sand better, so the mixture flattens out and flows.
Ivory is not the same as Orvus. Ivory is a synthetic fatty acid soap with additives (blueing, fragrances, etc.). Ivory is a very mild soap, so it is suitable for bathing pets and children. But the additives, not to mention the soap residues, are not good for needlework.
Most soaps also have molecules in them that attach to oils. Detergents mostly work this way but also have other ingredients - - to whiten, brighten, keep dirt from redepositing, enzymes to break up organic soils, and the like. The added ingredients make these products hard to rinse out.
This is mb again:
For more information on washing needlework, see this file.