Dr. Stone: How Did Senku Make Soap?


Is it just me, or is thinking about the discovery of soap in the stone age impossible? Making Senku Cola, okay I can see that it is kinda possible as most ingredients are present at the hand’s reach all they needed was imagination. But Stone age people barely knew how to feed and protect themselves from the wild beasts let alone making soap from nothing! It requires the power of Alchemy!

Even if a genius were to be born in that age or some nerdy ape man-made it accidentally,  it would be too difficult to trace back the steps for a monkey brain.

But in the world of Dr. Stone everything is possible with the diligent application of Science. Even though the world reverted back to the starting point of human civilization, Senku Ishigami has the answer for every science question. And with that, he made possible the discovery of Soap!

How did Senku make Soap?

Main uses of calcium carbonate in Dr. Stone
Main uses of Calcium carbonate in Dr. Stone

As we can recall in Episode 2 of Dr. Stone, soap is something Senku made as one of the first scientific items that made Tsukasa aware of Senku’s potential as a scientist. Among the four things he mentioned to make out of the calcium carbonate he acquired from the seashells, soap was third in the line.But we didn’t really get to see him make it on screen or in the manga. 

So allow me to explain the exhilarating stuff that Senku kept as a secret! Soap making is not rocket science, however it requires a little sense of chemistry. The main ingredients are- Kelp/Seaweed and Calcium carbonate.

I don’t want to give a chemistry lesson here, it is a recipe which mere school kids can easily recreate(under adult supervision).

I am sure the below reactions are the steps to make a fine soap that saved the characters from “Game Over”.

Do not try this at home without supervision or ample precautions. The ingredients used may be harmful for your eyes and skin. Wash your hands with lots of clean water if you attempt this anyway!

reactions which senku used to make soap
How Senku Ishigami Made Soap

So, if I explain the above diagram, Senku made Soap in the following way

Make fine powder of Calcium carbonate(CaCO3) from crushed seashells.

Mix some water to the Calcuum carbonate poweder to create “slaked lime: (Ca(OH)2).

Use Seaweed or Kelps to get Sodium Carbonate

Mix Sodium carbonate and slaked lime to get the key ingredient of Soap, Sodium Hydroxide (NaOH)

Now, vigorously mix Sodium Hydroxide with any oil or animal fat

Allow the mixture to rest and then dry it for days. Tadaaaa! We have finally made soap!

Now, now. This soap is not the glossy smooth colourful bars we use while enjoying a bath. The soap made by this process is very crude with little to no froth in the making. It has no luster, odor or skin benefitting elements. It is just a rough bar made out of throwing in things to keep you clean by removing all the dirt and impurities from your body. An alchemy product to keep the doctors away! So, don’t get your hopes up thinking Senku made a scientific breakthrough or something. It is simple science!

  • I was under the impression that to make slaked lime (calcium hydroxide), you need to add water to burnt lime (calcium oxide aka “quicklime”). Calcium carbonate is the lime produce before it’s burnt (also found in seashells). It doesn’t create the reaction needed to saponify the oils in soap, but is often used as an additive to alter the consistency of pure soap, which is typically made from lye (either sodium hydroxide or potassium hydroxide) with water and oils/fats.

    I have seen a traditional method of making a water resistant whitewash, which resembles the soap-making method using quicklime (calcium oxide/burnt lime – caustic like lye) to produce the exothermic reaction with water and fat, but this isn’t used for cleaning bodies, but for applying a protective whitewash coating to buildings, and best applied in hot liquid form.

    Adding lime to a soap mixture would create calcium stew rate (same as soap scum) like the Tadelakt finish used on building surfaces. Calcium stearate is hard and insoluble in water, so great for coating stuff, but not for handwashing.

    It would have been easier for this guy to use what early humans did – Soda ash with water and fats. If it was for cleaning stuff.

    Perhaps I talk crap. But that’s my impression. Which would be a worry. Since I make my own soaps and soap finishes.

    I wouldn’t recommend it for kids. Dangerous.

  • In the past they used to mix ashes with animal fat to make soap, and there was only few countries who made scented soaps like in Nablus Palestine

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