Simple steps on how to calculate the Proportion of Lye (Caustic Soda) to different oils For soap makers.

Saponification is the chemical process where a lye  or caustic soda ( Sodium hydroxide) or Potassium hydroxide (Caustic potash) reacts with oils to form soap. Like I said in the previous post, for soap to be formed, it must undergo the process of saponification. I also mentioned that any oil can be used to combined with lye to form soap. Each type  oil has a different proportion of lye needed to saponify it.

There’s need for consistency in measurement. Never measure lye, oils and water in volume but rather it should by weight. When using weight, do not mix the method for example grammes and ounce at the same time. If you are using grammes, use grammes throughout your measurements. If you are using ounce, stick to ounce throughout your measurements.

Alot of persons will want to dive to the main aim of this write up ignoring the preliminary part. Please try to read through and digest the introductory part before reading the formula.
In order for you to know the exact amount of oil to lye and the amount of water to lye, you must have knowledge of the saponification value of the oil in question. The chart, below, lists an average of the known saponification values for specific oils. With saponification value, it is  easy to determine the exact amount of lye needed to completely saponify a measured amount of a specific fat, so there will not be any extra lye in the soap and the soap will not irritate the skin.

Lets get going………

Calculating Lye amount and Measuring Water
Water is needed to dissolve the lye (sodium hydroxide) and for the hydrolysis of the fats. Using too much water may produce soft bars of soap, may demand extra drying time, or may encourage rancidity. Because the water is used to dissolve the lye, the amount of lye will determine the amount of water. To calculate the correct amount of water, first determine the total amount of lye (sodium hydroxide). Divide the amount of lye by 0.3 and then subtract the amount of lye from the result.
  1. (Amount of Fat) × (Saponification Value of the Fat) = (Amount of Lye)
  2. (Amount of Lye) ÷ 0.3 = (Total Weight of Lye Water Solution)
  3. (Total Weight of Lye Water Solution) − (Amount of Lye) = (Amount of Water)
For example, to make just over 200g of kernel oil soap

  • Measure 200g of kernel oil and 35.54 grammes of lye because 200g kernel oil  × 0.1777, which is the saponification value for kernel oil, = 35.54g of lye. Check saponification chart below to get the saponifcation value for kernel oil.
  • Next, to calculate the correct amount of water: 35.54g of lye ÷ 0.3 = 118.47, which will be the total weight of the solution (lye and water). 
  • Subtract the weight of the lye from the solution (118.47 − 35.54) to get the weight of just the water, which equals 82.93g of water.

From the above, we can deduce that to prepare a 200g kernel oil soap, 

  • We will measure 200g of kernel oil
  • 35.54g of lye
  • 83.93g of water to dissolve the lye

Remember, after weighing the lye (sodium hydroxide) and water, always add the lye to the water; never add water to the lye.

Click here to download Saponification chart.

Have a nice day!

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