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Chemistry of traditional soapmaking

  1. Sep 1, 2012 #1
    I have two questions regarding the first stage of traditional soapmaking, which, according to a bunch of DIY websites, is to make "lye water" out of "hardwood ash".

    Firstly, the 'pedia article on "wood ash" says that "potassium hydroxide [aka potash lye] can be indirectly made from wood ash by the addition of calcium hydroxide [aka slaked lime]". However, none of the instructional pages mention lime at all, just the ash and plenty of water - preferrably rainwater or some other kind of "soft water", i.e. water which has few mineral impurities, according to my understanding. Presumably, then, the hydroxide comes from the water itself, rather than from something else in the ash or from trace substances in the water, in this case? If both methods work, what is the advantage of using the calcium compound, when it's bound to be harder to obtain than ordinary rainwater?

    Secondly, is "hardwood ash" preferrable simply because it contains more inorganics like potassium, and is this what made the wood harder in the first place, as the name suggests - or is it more complicated than that? In principle, can ashes other than those of wood be used for this process, or is there something special about wood ash?

    Case in point, there is a folk etymology that derives the word "soap" from the legendary Mount Sapo, a site at which Romans supposedly burned animals as religious sacrifices. The resulting ashes, unburned (fatty) animal remains, and water from a nearby spring or stream then naturally combined, resulting in the creation and discovery of soap. Some versions of the story, including the one I linked to, specify that the ash in question was wood ash from the pyres, while others specify that it was the ashen remains of the burned animals themselves. To be sure, animals do contain some potassium, too (~200g for an average human, apparently), so could that work?

    Thanks in advance for any replies! :smile:
  2. jcsd
  3. Sep 1, 2012 #2


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    Ash contains potassium oxide, K2O, which reacts with water producing potassium hydroxide. However, if some potassium is there in the form of potassium carbonate, adding calcium hydroxide should produce insoluble calcium carbonate and soluble potassium hydroxide - increasing yield. But that's just a guess.
  4. Sep 2, 2012 #3


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    That's a very good reason IMO. I've often seen the claim that wood ash contains potassium oxide but it is far more likely the potassium is present as it's carbonate. You will note that the article also states that the calcium component is present as the carbonate. This is obviously wrong for fresh ash but could be the case for ash left open to the air for some time. I believe the magic of producing potassium hydroxide from an intimate mixture of Calcium oxide and potassium carbonate (wood ash) occurs in the 'dripping' step. Water is added to ash, some time goes by during which a double displacement reaction occurs ( calcium oxide -> calcium hydroxide -> calcium carbonate ; potassium carbonate -> potassium hydroxide) and the soluble potassium hydroxide is filtered.

    Since it is likely that dripping lye was done with ash accumulated over a considerable time rather than from recently burned ash it is very reasonable to expect that calcium oxide in the ash has converted to the inactive carbonate. This would result in the dripping process being significantly contaminated with potash rather than the hoped-for hydroxide resulting in stoichiometry problems and the well known issues with potassium carbonate. Of course purpose-burned fresh ash won't have these problems so you see references in the old recipes for warm or fresh burned ash all the time. If you used old ash and obtained a 'drip' of unknown purity (carbonate content) you could fix it with a subsequent treatment of lime.
    Last edited: Sep 2, 2012
  5. Sep 2, 2012 #4
    Ah, I see, that makes more sense now. Thanks, both! :smile:

    One of the DIY sites I went through yesterday actually mentioned that their ash wasn't fresh, but that they'd stored it in an airtight container, because they'd heard that it would remain "active" that way. Which meshes perfectly with what you just explained.

    Any opinions on the non-hardwood/non-wood ash part of my question?
  6. Jan 7, 2013 #5
    Well it's been 4 months, so maybe this is a dead issue, but I thought I'd chime in:

    1) Ash is viable so long as it hasn't had water run THROUGH it. Ash has soluble and insoluble components, and you want the soluble ones for potash. So damp ash is ok, provided it hasn't accumulated and drained away.

    2) Hardwood ash has more K2CO3 than NaCO3. (More pearl ash than soda ash) If you were to use the ashes from burning grass and shrubs that flourish near the ocean, you'd get more Na than K. The preference for hardwood comes from two things: higher yield of ash per volume of wood burnt (than softwood) and more potassium than sodium (vs Barilla, etc)

    3) Talking about oxides rather than carbonates. Ceramics and concrete people always talk about "oxide content" rather than what's actually there, since at the end of the firing or curing, carbonates, hydrates, and whatnot will end up as oxides. It's just tradition. You need to mentally convert back if you want actual amounts of carbonate.

    4) Calcium oxide (quicklime) can be made from calcium carbonate at attainable temperatures (900C) while the dissociation temps for K2CO3 -> K2O and NaCO3 -> Na2O are much much higher. Thus a multistep process, driving off the CO2 from CaCO3, leaving CaO, mixing into water giving Ca(OH)2, and then a metathesis reaction giving KOH and CaCO3 is a simple (attainable temperature) path to get to KOH.

    Lastly, the story about mount sapo is questionable. Pliny discusses in depth the way the romans bathed: hot water and cold, oils, scraping tools, etc... but never mentions soap being used on people. If his descriptions are otherwise so detailed, it seems odd he'd leave that one (very important) part out. Many consider this a reason to question roman use of soap per se.
    Last edited: Jan 7, 2013
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