Adding salt during the soap-making process to make potassium hydroxide soaps harder

By Matthew of Summerdale

I took a class on saponification at the Summer 2023 Atlantian University by Lucia de Moranza from Ealdormere.  Taking this class led me to start to do some soap making.  While I have started with modern soap making methods and processes, I have started looking at period soap making.  One technique I found mention of was adding table salt during the soap-making process to make harder soap.  


Saponification is the chemical process that occurs during soap making.  It is done by adding a strong base to an oil or fat (Moranza, n.d.).  They are mixed to form an emulsion.  Modernly, the bases used are sodium hydroxide (NaOH), or potassium hydroxide (KOH).  These both are often referred to as lye.  Sodium hydroxide produces hard bars of soap, and potassium hydroxide produces a soft soap that is used to make liquid soap.  

In period, lye was produced by passing water through wood ash.  Using most wood ash would produce potassium carbonate dissolved in water.  This could be used to make soap, though it is less strong than potassium hydroxide.  Calcium hydroxide (quicklime) could be added to the potassium carbonate and would produce potassium hydroxide and calcium carbonate.  The calcium carbonate would precipitate out of the solution.

If the process of passing water through ash were done with ash from beach plants or seaweed, sodium carbonate would be produced instead of potassium carbonate.  As with the potassium carbonate, sodium carbonate could be used to make soap directly, or calcium hydroxide could be added to convert it to sodium hydroxide.  

I found mention that in period, salt was used as part of the process (Verberg, 2016).  I would expect soap made from potassium hydroxide and salt to be harder than soap made from just potassium hydroxide.   Because there would be sodium ions in the solution, in addition to the potassium ions, the saponification process would behave like there was a mix of sodium hydroxide and potassium hydroxide, and result in a harder bar of soap.

Experiment Process

To conduct the experiment and see if adding salt made the soap harder, I made six batches of soap.  First, I chose 2 different fats to use separately, beef tallow and olive oil.  I chose these because they were fat sources that would have been available in period and were used for soap making.  I chose to use 2 different fats because the tallow is solid at room temperature and the olive oil is liquid, so I wanted to see if that made a difference in the results. I would expect a harder bar from the tallow, as it was already harder.  With each of those fats, I would make 3 different soaps, one using sodium hydroxide, one with potassium hydroxide, and one with potassium hydroxide and salt.  This way I would have some soaps to compare the KOH with salt soap against, so I could see if it was harder than the KOH soap and if it got as hard as the NaOH soap.

I used the calculator at to determine the formulation for making the soaps.  The recipes I used are attached.  The batches were based on 8oz of fat with 3% superfat.  Superfat is fat in excess of the amount of KOH or NaOH used. For the batches with salt added, I added the salt at 15% the weight of the oil, so 1.2oz of salt.  The 15% salt was based on a recipe I found on a soap making forum from someone who said it had worked for them (Potassium hydroxide bar soap, 2016).

I hot processed the soap using a crockpot.  I have found some mentions that soap in period was made in a form of hot processing (Verberg, 2016).  It is an area for future research on the exact process.  I combined the lye with water and added it to the fat.  I then blended them with an immersion blender until trace was reached.  Trace is when an emulsion has been created and the mixture has thickened up.  The soap was then cooked until it reached the wet mashed potato state, and then was put into a mold.  The mold was a mini loaf pan lined with parchment paper.

The soaps were made over several days at the beginning of September 2023, on the 1st through the 4th.  This allowed for cleaning of equipment between uses but kept the age of the soaps close to each other for later comparison.


The soaps made with sodium hydroxide firmed up quickly in the mold and could be removed from the mold the next day.  This was the expected behavior for soaps made with sodium hydroxide.  

The soap made with potassium hydroxide and tallow has become somewhat hard.  The soap made with potassium hydroxide and olive oil is still soft at the end of September and is more of a paste.  I have not removed it from the mold out of concern that it would ooze out into a puddle.  I flipped it over in the mold to attempt to allow it to dry more.

The soaps made with potassium hydroxide and salt are noticeably harder than the ones made with just potassium hydroxide.  The olive oil, potassium hydroxide and salt soap was harder after half a day in the mold than the olive oil and potassium hydroxide soap that had been in the mold for 2 days.  

After sitting for about a month, the soaps are largely unchanged.  The sodium hydroxide soaps are still hard.  The potassium hydroxide and salt soaps are still firm.  The potassium hydroxide tallow soap is somewhat firm but is a bit soft.  The potassium hydroxide olive oil soap is still more of a paste.  


Adding salt to soaps made from potassium hydroxide makes them harder, close in hardness to soaps made from sodium hydroxide.  The effect is more noticeable in soaps made from fats that are liquid at room temperature versus fats that are solid at room temperature.  I plan to do some further experiments involving varying the amount of salt used to see if there is a gradual change in hardness as the amount of salt used is increased or if there is an amount of salt that causes an abrupt change from soft soap to hard soap. 


Moranza, L. (n.d.). Alchemy 201: Saponification.

 Potassium hydroxide bar soap. (February 24, 2016). Natural soap and home made products.
Verberg, S. (September 2, 2016). Modern & medieval soap making compared.

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