How to clean old barnwood?

I have salvaged some old barnwood from the mid 1800s. It has a beautiful silver patina that I do not want to strip away. However, the wood is very dirty. How should I go about cleaning it? I do not plan to sand or stain it, just use a simple water based poly to seal it. Maybe not even that. I want to leave it as natural as possible, minus the grime & dirt. Also, power washing is not an option, as it would destroy the wood. Thanks!