Formula for a "sludge wash"

I am ready to try weathering an aircraft model and have decided to try a sludge wash. I remember an article in an old issue, but I can't look it up as all my magazines are still in storage due to a house fire a while back. Can anyone help me out here? I think it called for acrylic paint and dish soap, but I can't remember if there was anything else or how much of each. Thanks much.