To Make Black Sope

What is soap?  When a strong hydrolyzing base such as lye comes into contact with a triglyceride like tallow, soap is formed alongside with glycerin. A soap molecule has a long fat-loving tail and a water-loving head which makes it possible for the molecule to bond to both water and to grease, and thereby dissolve fats and oils into water. Sodium hydroxide (caustic soda, soda lye, NaOH) makes solid bar soaps, while potassium hydroxide (caustic potash, potash lye, KOH) makes liquid or soft soaps. This is because of the difference in their molecular structure: sodium hydroxide is a small molecule with short and tight molecular bonds, creating soap with a strong crystalline structure. Potassium hydroxide is large molecule, with long weaker molecular bonds, creating soap with a weak crystalline structure. On average, the pH of soap is between 8 to 10. Human skin is around pH 5.5. The higher the pH of soap, the harsher the soap will be on the skin.

What is medieval soap? Medieval soaps are made with drip lye, and are either Black Soap, which is a soft potassium based soap, or White Soap, which is a hard sodium based soap. In early Renaissance, soap balls became in fashion which were made from store bought grated hard soap kneaded with botanicals and scents. The lye to make soft and hard soaps with was made by leaching water through ashes; hardwood ashes for potassium hydroxide lye, and marine plant ashes like barilla for sodium hydroxide lye. Ashes are highly concentrated minerals of hydroxides, nitrates, carbonates, sulfites, etc. depending on how well it is burned. The completer the burn of the plant material the higher the pH of the lye; in case of less complete burns like in fire pits and fireplaces lime would be added to help change left over carbonates into hydroxides to thereby raise the pH of the lye.

Lime, quicklime, unslaked and unquenched lime is all the same for calcium oxide, before oxidation into calcium hydroxide. Slaked and quenched lime is already oxidized into calcium hydroxide. As calcining is not a difficult process, and as lime is easier (or cheaper) to get than calcium hydroxide, many historic recipes use straight lime. Drip lye has a pH of about 11 while modern lye has a pH of 14 – one big reason why making medieval soap is so different from modern soap-making.  Handling drip lye therefore is also a lot less dangerous than handling modern lye, but just in case, do keep some vinegar handy – acid on base neutralizes like a charm.

Modern hand-made soaps are most often Cold Process soaps; early medieval soaps are most often Hot Process soaps, and this difference can be confusing for the re-enactor. In hot-processed soaps the saponification reaction – as the chemical reaction between lye and fat is called – is accelerated by adding heat. In contrast; cold-process soaps are poured into molds and with added insulation or a little radiant heat only then do they saponify.  Hot-process soaps saponify completely during cooking and only then are poured into molds. Hot-process soap making also differs as it allows the soap to be fully saponified and almost immediately ready for use after cool down instead of having to cure for weeks, as the cold-process soaps requires.

Medieval hot-process soap cooking was quite a bit more challenging due to the unknown strength and quality of the lye. The purity of the alkali lye was unreliable, as only naturally found alkalis, such as wood ashes, were available. The advantage of hot-process is that the exact amount or purity of lye required for successful saponification is not needed as slight changes to the ratio of lye or oil can be made during cooking. There was a real need for good soap and a surprising lack of knowledge on how to properly make it throughout much of medieval Europe. It was not until pure crystallized sodium hydroxide was invented and patented by French chemist Nicolas Leblanc in 1790 that exact measured recipes with predictable results were possible.