Thursday, February 6, 2014

Historical Lye Making, Part 1

One thing that's not done as much anymore as it should be is making lye on the homestead.  A major contributor to this phenomenon is likely the abundant scary stories and mystique of danger surrounding lye because of some horrible accidents in the past and a general fear of the unfamiliar.  This isn't to say that lye isn't dangerous--it certainly deserves a healthy respect and some reasonable precautions--but, like most things, having an awareness of the properties and dangers is a better approach than running away screaming or cowering in the corner like a congressman.

Ok so, newly emboldened about the utility of lye, let's take a look at why you would want to make your own.  Other than not having to buy an ingredient for your soap-making days, starting with a potassium-based lye instead of a sodium-based one (typically what's available commercially) makes it easier to recycle the soap as part of your graywater scheme since you don't have to worry about sodium buildup.  (Plants need more potassium than sodium.)   In addition to soapmaking, you'll save on ingredients for your homemade drain cleaner, biodiesel (if you get really good at making lye), and lutefisk recipes.

Back in the olden days, when soap making was a standard activity on the homestead, the source of lye was normally wood ashes.  (Even back then, folks knew that grass ashes (e.g., from corn cobs) gave more lye than wood ashes, but no one burned grass in significant quantities.)  The goal was to leach the soluble lye, mostly potassium hydroxide (KOH) in this case, out of the wood ashes, leaving the insoluble parts behind and obtaining a concentrated lye solution.  More generally, however, wood ashes were leached to collect potash: an umbrella term for soluble potassium salts, which could contain potassium hydroxide (KOH), potassium carbonate (K2CO3), potassium chloride (KCl), etc.  The potash was commonly evaporated to dryness and sold as fertilizer when it wasn't used to make lye for applications around the homestead.  And, as with most old homesteading practices, there is an interesting confluence (to us, anyway) of chemistry and history around the lye-leaching process.

Traditional lore says that lye should be leached from hardwood ashes, especially hickory. The reason hardwoods, and especially hickory, should be the best has puzzled us for a long time, since modern methods suggest that softwoods contain no less potassium than hardwoods, at least inherently.  However, old sources also show some data confirming that indeed, less potash is obtained from softwood ashes than hardwood ashes.  Why should that be the case?  We can think of two possible (but contradicting) explanations, neither of which we can find confirmation for.

First, many sources assert that softwoods contain more resins (although we haven't been able to find any quantitative measurements), and thus burn hotter than hardwoods.  The modern source linked above shows that ashes from wood combusted at temperatures above 900 °C lose a significant amount of potassium to evaporation.   Thus, if softwoods burn at 950 °C and hardwoods at 700 °C, softwood ash would likely contain less potassium than hardwood ash.  However, 900 °C is a very high temperature, especially for the open-air fires common before the turn of the last century.

We think it's more likely that softwoods don't burn as cleanly, and more of the inherent potassium remains locked up in the incompletely burned remains.  Some experimental evidence from the period suggests that leach-resistant potassium can indeed be found in incompletely burned softwoods.  (On the other hand, that may mean that softwood biochar is more beneficial for the garden than hardwood biochar as a slow-release potassium fertilizer.)

In any case, it seems legitimate that hardwood ashes are preferable to softwood ashes for making lye, and a clean-burning, fairly hot fire (but less than 900 °C) is the best way to get those ashes.

Also, some old lye recipes call for adding lime or slaked lime to the ash-leaching barrel.  The reason for this addition is clear:  in water, lime (calcium oxide, CaO) becomes slaked lime (calcium hydroxide, Ca(OH)2), which reacts with potassium salts, such as potassium carbonate (K2CO3) to form calcium carbonate (CaCO3) and potassium hydroxide (KOH, the lye we want!).  Depending on conditions, however, the majority of potassium may be leached as the hydroxide anyway, so the lime may only give an incremental increase in lye yield.  Many lye makers don't bother with the lime and still make fine soap, so it seems that the lime must be optional.  For fancy-pants lye making only, if you will.

Additionally, lye leached in traditional ways often times comes out transparent-ish, but very brown-colored.  The reason is that the layers in a lye-leaching bucket normally included a layer of sticks, a layer of straw, and then the ashes.  The lye leached from the ashes can start to decompose the straw and/or sticks, which yield the brown-colored compounds (primarily from solubilized lignin components).  Leaching the lye through a different material, like a tightly-woven t-shirt (multiple layers), or leaching through the straw so many times that all the brown parts are dissolved, would probably yield a clear lye solution.

Finally, many sources indicate that lye should be leached from ashes using distilled (or rain, or soft) water.  For leaching lye per se, water hardness shouldn't make much difference (see paragraph above about adding lime), but if you plan to make soap from the lye down the road, it will be beneficial to not have the hardness in the water.  The cations in hard water are divalent (+2 charge), which means they will take on two soap molecules, become essentially nonpolar, and precipitate out of the aqueous solution, almost exactly like a Dementor eating someone's soul (if one soul = two soap molecules).

The setup for a lye-dripping (leaching) trough, as described in several old books.  It could also be a barrel with a plug in it.  Don't forget to put a bucket under the arrow, or your lye will run out onto the ground.  For other setups, see here, here, and here.

 After leaching the lye, it should be tested for strength.  There were a number of traditional methods, including floating eggs or potatoes, dissolving feathers, and making sure it tastes incredibly bitter.  (Don't try the last one).  Modern techniques include testing the pH and/or measuring the density (the latter being an updated version of the egg or potato test). 

Since this post is already very long, we'll wrap it up here.  Check back on Sunday for Part 2, featuring lots of pretty colors!

In the meantime, have you dripped your own lye from ashes?  What did your setup look like?  Do you have a better idea why hardwoods are better than softwoods for making lye from the ashes?  Let us know in the comments section below!


  1. make lye 3 - 5 hour

    make the concentration of alkali in a good quality, if you want to be able to fast results is wearing electrolstyc.

    How: mixed wood ash ( ash from burning rice husk / wheat) with distilled water, then input du pole anode and cathode in the solution.
    Of two valves anode and cathode we can see the water bubble that glows almost the same.
    Water into the ash-gray turned to white, then change color to green again, then turns dark yellow, as well as the wave radiated from the two poles is lost in solution.

    Voltage of 12 volts at 5 amperes power supply, it takes 2-3 hours in a standard cup of coffee.

    If the lye is mixed with sulfuric acid, with a ratio of 1: 1/2, the color of the solution turned into a clear blue color, and if we put a potato / egg will float above the liquid

  2. For making my own lye in a smaller batch I cut the bottom of a glass carboy to make a glass funnel, and suspended that upside down in a rope harness from a cooking tripod, with a deep dish to collect the lye.

    I used folded cheesecloth tied around the mouth of the carboy as the filter, no straw or pebbles ore anything else. The lye still came out real dark. After a second filtration the lye was much lighter in color.

    We use mostly hardwood in our stove, but know from experience softwood burns much quicker and hotter than hardwood. I attributed that to resins especially in the case of pine, but also larger cells in softer woods, making for easier burn than the very small compact cells of harder woods. One experience that underlined that was burning black locust, the stuff was impossible to get going but once it did it burned fine (same with trying to cut it, it's very hard).

    Do you have any experience making sodium hydroxide lye from sea plant ashes, like the Celts were known for?

    1. Hi suus, thanks for stopping by! That's interesting that you still get dark-colored lye when filtering through cheesecloth, which should be pretty much pure cellulose. It might be, as you mention below, that incompletely burned compounds from the wood are carrying through. I know that in other cases, colored compounds can be removed by passing the solution through a bed of activated carbon, so perhaps filtering through biochar would also work.

      I haven't tried making lye from sea plant ashes, but that sounds like a fun thing to try if we ever have an opportunity for an extended stay by the ocean!

  3. I wonder if there is a connection between the whiter the ashes the clearer the filtered lye. I found that without using any organic filtering materials the lye still filtered dark, like rooibos tea, but I am using a mostly white ash. Not all of my woodstove ashes are white, some is grey to dark grey, and I have a hunch the color might come from that (partially burned organics, like you mention) which is why using white ash is so important - white ash equals whiter soap.

    1. Could be! Lots of things are soluble at very high pH, so it's definitely plausible. If only we had a budget for an analytical chemistry lab on the homestead! :-)