Stick a Feather in and Squeeze... testing the strength of lye.

"the most difficult part of early soapmaking was determining if the lye was the correct strength. the “lye water” was considered the proper strength to make soap when an egg or small potato placed in the solution floated about halfway beneath the surface of the solution. if the egg or potato floated on top, the lye was too strong. if it sank quickly, the lye was too weak. some early soapmakers used goose or chicken feathers to test their lye. if a feather inserted in the lye water began to dissolve in it, then the lye water was at the right strength."
From http://www.ecobeings.com/soap-information/history-of-soap

"Unfortunately, we only made around a quart to begin with, so we'll only be left with a few drops at the end--not enough to do much with (even dissolve a feather, which was another test of lye strength we were going to try)."
From http://homesteadlaboratory.blogspot.com/2014/02/historical-lye-making-part-2.html

"Lye Water Strength: If an egg or potato will float just below halfway, or a chicken feather starts to dissolve in it, then the lye water is at the right strength."
From http://journeytoforever.org/biodiesel_ashlye.html


As you see, the idea of using a goose feather is prevalent on blogs and websites talking about historic drip lye soap making. But I've never been able to place the technique and further back than hearsay US Colonial times... until I was browsing through a late medieval German spot removal manual which has a couple of oddball recipes in the back, including To soften horn so much it can be formed into forms. And low and behold, there was a mention to using a feather to test the strength of lye, including how to use it! I'd tried it myself and from just sticking it in and looking nothing much happened (except leaving it in overnight resulted in some gorgeous crystals forming on the edges). I am looking forward by the next batch of fresh lye to go get a feather from the backyard, try it this way, and see what happens!


To soften horn so much it can be formed into forms. 
Take one pound of the ashes used in making glass, one pound unslaked lime, one "mass" water, allow to boil together until two thirds are evaporated, then stick a feather in and squeeze it between two fingers: if the hairs come off, the boiling has been enough, otherwise let boil longer; let it clarify and pour off, put in small chips of horn and let soften for two days, smear oil on your hands and the horn to make a paste and press into what you wish.

From the Allerley Matkel (1532) by Sidney Edelstein, Technology and Culture, Vol 5 no 3 (Summer 1964), pp. 297-321 - includes the original fascimile text with a translation and discussion.