But isn't potash soap harsh?

Many Colonial recipes for making drip ash lye mention the strength of lye to be floating an egg with a quarter of its shell showing (or the size of a quarter coin), and this is the soap re-enacters complain of as being so harsh (and throw away after the demo...). Interested in making potash soap myself but wanting to use medieval techniques this test proved intriguing, very useful, but regrettable out of period, and on the wrong continent, at least at first... Then by way of word of mouth - word was getting around I collect soap recipes - an acquaintance sends me a copy of an entry in an 18th century Encyclopedia Brittanica. And it mentions using an egg to test the density of the lye! So now I had it at tracked back to the right continent. The final puzzle piece was uncovered when I spent a couple mornings at Cornell to dig my way through the Early English Books Online catalog and came across the manuscript The seconde part of the Secretes of Master Alexis of Piemont by hym collected out of diuers excellent authours, and newly translated out of Frenche into Englishe, with a generall table, of all the matters conteined in the saied boke. by Italian writer Girolamo Ruscelli.

This Book of Secrets from 1560 mentions lye strong enough to float an egg: “the magistrale lie commeth out, put the Egge into it, and whiles the egge remaineth aboue, put it al into a vessell, for it is the first whiche you ought to make muche of”. This makes Laundry Soap or Black Soap and is meant to be harsh to better to clean cloths with. It is also harsh on the washed fabrics resulting in wear and tear and if the garments are not rinsed well soap remnants in the clothes are known to itch… good reasons why wealthy households would buy their soap and not make it themselves (difference between just harsh enough and too harsh). This does not mean all soft soap is harsh, only that laundry soap recipes make harsher soaps.

To make black Sope for clothes, with all the signes and tokens that it giueth and maketh in beiling. [...] and bicause you maie the better knowe the firste, the seconde and the third, take an Egge newe laide, and binde it rounde about with a threede and as the ma|gistrale lie commeth out, put the Egge into it, and whiles the egge remaineth aboue, put it al into a ves|sell, for it is the first whiche you ought to make muche of. And whan the egge sinketh in the lie [suspended], put that se|cond by it selfe: and if you can gette of the first fourty pounde, you shall get of the seconde thirtie, and of the thirde twentie [touching the bottom], and of the fourth asmuch as you will: [pretty much the density of plain water] [...]

But does all potash soap have to be harsh?
Another example from the 1558 book of The secretes of the reuerende Maister Alexis of Piemount Containyng excellent remedies against diuers diseases shows the following recipe for shampoo. It lists the strength of the lye “as stronge lye that will beare an egge swimminge betwene two waters” which I interpret to mean suspended in the middle. As suspended-egg lye makes neutral soap which does not ‘bite’, this makes sense as this is a shampoo soap meant for personal use.

A very exquisyte sope, made of diuers thinges.
TAke Aluminis catini thre vnces, quicke lyme one part stronge lye that will beare an egge swimminge be|twene two waters, thre pottels, a pot of commun oyle: mengle all well together, puttinge to it the white of an Egge well beaten, and a dysshefull of the meale or floure of Amylum, and an vnce of Romayne Vitrioll, 55 or redde leade well beaten into poulder, and mixe it continuallye for the space of three houres, then lette it stande, by the space of a daye, and it will bee righte and perfite. Finallye,  take it oute, and cutte it in pieces: af|ter sette it to drie twoo daies, in the wynde, but not in the sunne. Occupie alwaies of this sope, when you will washe youre head, for it is verie holsome, and maketh faier heare. (Ruscelli 1558)

So no, potash soap does not have to be harsh - it can be as harsh, or soft, as you want it to be! Experiment, take notes, and before you know it all your friends will happily enjoy a bowl of home made scoop-able hand soap for by their kitchen sink!