Occasionally one of my main characters has to wash a dish or two. (Remember, I write American Historicals - not the regency romances where the servants do all the housework!)
To help pull my readers into the setting, I did a little digging on dish soap in the 18th century. Surprisingly, I could find very little on the subject. There was plenty written about making laundry or bath soap (apparently, they were pretty much the same thing) but very little about what one might use to wash dishes.
I posed the question in one of the history discussion groups on LinkedIn and they agreed that little was written about dish soap. We pretty much came to the same conclusion - hot water and maybe a scrubbing agent like sand was enough to do the trick. One commenter reminded me that it's not a good idea to wash cast iron in soap as it ruins the "seasoning" - that black crust that novice cooks always try to scrub off. I know that when helpful relatives visit and try to use soap on my cast iron cookware, it takes forever to get rid of the soapy aftertaste.
Also, it got me thinking back to my girl scout days when doing dishes meant scraping your plate and then sticking it in a big kettle of water that we boiled over the fire. As I recall, we had one for washing and a second one for rinsing, but no soap was involved.
If any of you know more about what the 18th century American woman (or I suppose the 18th century American man who was way ahead of his time) might use to wash a dish, please let us know.