Good A&S Documentation - Castile Soap

Entry: Castile Soap
By: Caitilin the Naughty
MKA Debbie Quinlan

My entry is a soap made with 100% olive oil, lye and water. This would have been akin to those produced in the 12th century in Italy (1), and which was traded with England in the 16th century (2).

The entry consists of one bar of soap. This is an off-white colour, with a slight greenish tinge. It has lighter markings.

The history
Soap has been around for centuries and is documentable back well before the dates given above (3), however, most soap was liquid or soft. This was because it was made with potassium hydroxide (4). The French however, discovered that using the Barilla plant to create ashes, and therefore lye water, created a product more akin to sodium hydroxide than potassium hydroxide. This is because it his high in sodium. This yielded a harder bar. (5)

These bars where presumably also traded, as they are referenced in several English works from around the sixteenth century.(6) It is unlikely the bars where produced locally for several reasons. To begin, Britain had no olive orchards. As a second point, the Barilla plant did not grow in Britain, thus severely limiting their ability to produce hard bars. For that reason, I believe they traded with the French and/or Spanish.

The science
Hard soap is created by the chemical reaction between a strong alkali (lye) scientifically known as Sodium Hydroxide and the fatty acid molecules in oil or fats. The lye is dissolved in water to allow for an even emulsion to be created in which an exothermic (heat creating) reaction takes place.
Three fatty acid molecules are tied together into a triglyceride molecules, and combine with three sodium hydroxide molecules to form 3 soap molecules and 1 glycerine molecule. The soap is still relatively soft due to a high water content that can evaporate harmlessly. The ph value of skin is 7.2, and the closer the soap is to that ph value, the milder it is on the skin. Commercial soaps are around a ph of 9 to 10, the higher the ph value the harsher the soap is. (7)

The historical methods
Historically, olive oil would be macerated and then pressed until the oil was extracted using mortars or beam presses. The best of the oil would be used form human consumption, the rest of the oil would be strained and used in cosmetics and lamps. (8)

Lye would be made by burning wood, or in this case the Barilla plant to create ashes. Water would then be poured over these ashes, and in some cases left to soak until it was a dark strong lye water. This would then be poured off and mixed with the oil, over a fire and thus soap would be created. (9)

The method used
As I have no olive grove, I bought olive oil from the local store. I purposefully did not buy extra virgin olive oil, or any special oil, but the lowest grade oil there, as this would most resemble the last pressing of the olives.

For lye, I bought Sodium hydroxide, as the method described above would not allow me to accurately estimate the strength of the lye and for health and safety reasons, I decided I would rather have a strong caustic substance where I could control the quantity, than one where I could not.

Having both the olive oil and the lye, I used a saponification calculator available online to double and triple check my calculations as to the amounts required. I then adjusted these to be olive oil heavy, as I am currently assuming that soap sold as a luxery bar would not have been harsh to the skin. With the trouble measuring the strength of the lye, it is likely the soap would have been oil heavy. I then measured out the water, added the lye while stirring slowly (and not breathing in the fumes as they are toxic) and set the jug to cool. I heated up the measured olive oil until it was within five degrees of the lye and then poured the lye in while stirring slowly. I kept the pan on the hob on a low heat for several hours while stirring and when it appeared to form a trace (an indented line when pulling the spoon through the mixture) I poured it into a mould

I then left it for three days, and unmoulded it, cut it, and left it to cure for a month in a shady and temperate shelf. After testing with Litmus paper, the ph value is 8.

My recipe
Olive Oil: 2777.0 grams (no rounding)
Lye: 258.7 grams (rounded down)
Water 669.2 grams (rounded up)
Effective percentage extra oil: 3.6%

This was based on:

Mappae Clavicula
Spread well burnt ashes from good logs over woven wickerwork made of withies, or on a thin-meshed strong sieve, and gently pour hot water on them so that it goes through drop by drop. Collect the lye in a clean pot underneath and strain it two or three times through the same ashes, so that the lye becomes strong and colored. This is the first lye of the soapmaker. After it has clarified well let it cook, and when it has boiled for a long time and has begun to thicken, add enough oil and stir very well. Now, if you want to make the lye with lime, put a little good lime in it, but if you want it to be without lime, let the above-mentioned lye boil by itself until it is cooked down and reduced to thickness. Afterwards, allow to cool in a suitable place whatever has remained there of the lye or the watery stuff. This clarification is called the second lye of the soapmaker. Afterwards, work [the soap] with a little spade for 2, 3 or 4 days, so that it coagulates well and is de-watered, and lay it aside for use. If you want to make your soap out of tallow the process will be the same, though instead of oil put in well-beaten beef tallow and add a little wheat flour according to your judgment, and let them cook to thickness, as was said above. Now put some salt in the second lye that I mentioned and cook it until it dries out, and this will be the afronitrum for soldering

The method I used is far from medieval, as I used modern ingredients which are mass produced. However, considering this was done for health and safety reasons, I believe this is justified.

The soap itself is good, and I have been using a bar for several weeks. It does not lather as well as commercial detergents, but it also does not contain the chemical additives that they do. I appear to have stirred unevenly, as some parts of the soap where slightly unevenly saponified, and this created white flecks in the soap. It is lighter than it would have been then, as ashes would have made a very brown lye water and sodium hydroxide is essentially white nowadays.

Overall I would probably try and blend the mix better, or heat it slower to prevent this happening in future.

1) This is dated by the "Mappae Clavicula: A Little Key to the World of Medieval Techniques." This is a book from around 1130 from Italy. Source:

2) This is dated by Delights for Ladies, Sir Hugh Plat,1609, and The Queen's Closet opened (1655), both English productions that use Castile soap.

3) The earliest known soap-like material has been found in clay jars of Babylonian origin dated around 2800 B.C. encyclopedia Britanica

4) 1790 Nicolas Leblanc made sodium hydroxide from salt
previous soap was made with lye water made with water-soaked ashes. Encyclopedia Britanica.

5) Making Soap with James Herschberger, a Chemical Engineer.

6) Delights for Ladies, Sir Hugh Plat,1609,
The Queen's Closet opened (1655)

7) ph 6.5-7.65 Human skin, saliva and blood
Ph 9-10 soap

8) historical olive oil production:

9) historical lye production