Historical Lye Making Part 2

On Thursday, we posted about historical methods to drip lye and some of the chemistry associated with it.  Today, we wanted to talk about measuring the strength of the lye to check if its concentrated enough for making soap.  In the olden days, that was often done with some sort of density test, either by floating an egg or potato, or matching the density of the lye solution with a saturated table salt solution (sodium chloride, NaCl).  An egg has a density of 1.03-1.1 g/mL, and a saturated NaCl solution is a little more precise at right around 1.2 g/mL, but in our soap calculations, we normally use a solution that is 1.3 g/mL, according to density-sodium hydroxide concentration correlations.  One initial conclusion from that is that old-time soap makers probably used less concentrated lye solutions.  Similarly, older soap recipes often call for cooking the lye water and fat together (i.e., hot process soaps), which probably boils off a lot of the extra water.

For modern homesteaders, who might have a scale and measuring cup handy, it would be much more precise just to measure the density of the lye solution directly.  (A graduated cylinder would make this calculation--and other density calculations you might want to do--more precise, but a measuring cup, used judiciously, should be good enough.

Another technique is to use a pH indicator and either dilute a small (representative) portion of the lye water or titrate it with an acid (such as vinegar) to find the strength of it.  The pH indicator would also be useful during the soapmaking process to check the progress of the saponification reaction.

Lets take a look at each of those techniques in more detail.

To make soap, we normally use a ratio of something like 2.89 oz NaOH to 7.87 oz water, which works out to about 26.9 wt% NaOH.  According to the above calculator, that should give us a solution density of 1.29 g/mL.  (The analogous numbers for KOH lye would be 4.06 oz KOH to the same amount of water, giving 34.0 wt% KOH, and a density of 1.33 g/mL.)  If we take an egg density of 1.1 g/mL, we can calculate the amount of water that should be displaced by the egg if we know the volume of it.

A typical large egg has a mass of 57 g, corresponding to a volume of 51.8 mL.  Buoyancy dictates that the egg should displace 57 g of the lye solution, which will correspond to a volume less than 51.8 mL if the lye solution is more dense than the egg (which it should be if the egg is floating). As an approximation, we can find an equation for an egg and make a graph to see how much of the egg should be above the water for a quarter-sized interface.

The egg equation came from here, but we normalized it to match the dimensions of an actual egg.  We assumed that an egg was sufficiently symmetrical to use a 2-D projection and calculate areas instead of using a 3-D model and calculating volumes.  In reality, the egg will sit with the skinny end slightly lower in the water since the air pocket is toward the flatter end.  In any case, leaving an area the size of a quarter above the surface would require a lye density of 1.13 g/mL, which is considerably less dense than our standard recipe, which has a density closer to 1.3 g/mL.  If the egg were a little less dense (toward the 1.03 g/mL end), it would sit higher.  As a point of reference, a potato has a density near 1.09 g/mL, in the same range as an egg.

This is a real egg in our standard lye solution (using NaOH).  The solution is yellow because we were testing pH indicators with it (described below).  The real egg looks not too far off of the graphical one, but there are more precise ways to test the lyes strength. 

For example, using data found here (and their related NaOH calculator), we can make a correlation, measure the density of the lye directly, and use the correlation to calculate the concentration.  It would help to have a digital scale and a graduated cylinder, but you can probably get at least as close as the egg/potato method with an old spring-loaded scale and a measuring cup.  We dripped a small batch of lye recently and were doing tests with it, but accidentally spilled it in the kitchen sink before we could test this method.  For lye dripped from ashes, use the KOH equation.  Note that if our solution density is 1.3 g/mL, our lye concentration (as KOH) is about 34 wt%, or 5.2 molar.  This density method will be our favorite lye strength test going forward.

Another way to test the strength is with a pH indicator.  One natural pH indicator is cabbage juice, which contains anthocyanidin pigments.  (As an aside, we noticed similar color changes in elderberry juice and wondered why; elderberries have a similar set of pigments.)  These pigments change structure as the pH of a solution changes, with each structure having a different color.  See here for more info.

The pigment structures of the cabbage anthocyanidins look something like this, with the different colors as shown.  Part of the reason the change from red to purple happens over such a wide pH range is the colorless intermediate.  Similarly, the yellow compound starts to form at pH > 8, but doesnt become the dominant form of the molecule until much higher pH (the presence of both yellow and blue make the solution green, kind of like a Ziploc bag).  The "R" groups are glucosides (i.e. substituted glucose molecules).  Sources for this figure came from here, here, and here.  If you took note of the concentrations above (i.e., that our standard soap recipe calls for 5.2 molar lye) and you are familiar with the pH scale, you might realize that theres a bit of a problem here.  That is, our lye should be at pH 14.7, but our indicator will be yellow at every pH > 11.

Fortunately, we can dilute a small, representative portion of the lye to bring it into the pH range where the indicator is effective.  On the far left in this picture is an undiluted lye solution we dripped from some wood ashes a few weeks back; its yellow, which means the pH is at least 11.  Since pH is measured on a log scale, diluting by a factor of 10 (conveniently 1 teaspoon solution plus three tablespoons water) should decrease the solution pH by one unit (assuming the water is actually neutral).  On the first dilution, the solution is already green!  That means the undiluted solution was not much over pH 11.  The further dilutions (using one teaspoon of the first dilution plus three tablespoons water, etc.) are consistent with that conclusion, looking similar to pH 9 and pH 7-8  solutions above.  The upshot of this technique is basically (heh) that if the lye is concentrated enough for soapmaking, it should take at least four 1:10 dilution steps to show a color other than yellow.  Alternatively, that means that we should concentrate our lye solution by a factor of 1000 before using it to make soap.  Unfortunately, we only made around a quart to begin with, so well only be left with a few drops at the end--not enough to do much with (even dissolve a feather, which was another test of lye strength we were going to try).

Another approach would be to titrate the lye with an acid, and figure out how much acid we needed to observe a color change.  Maybe that will be the subject of a future post.

Guess well just have to dry it down with waste heat from the oven (after baking bread or something) and store it in a jar until we can make some more!

Also, in case youre interested, heres how we made the pH indicator solution.  We chopped about a third of a cabbage to give around four cups chopped cabbage.

Then we poured about two cups boiling water onto the cabbage and let it steep for about two hours.

Then we strained out the cabbage (and made coleslaw!), leaving this dark purple-colored liquid.  We add about a teaspoon of this cabbage tea to a cup of liquid to test the pH. Its a little-known fact that a hot jar of this liquid was the inspiration for both the band name Deep Purple and their hit single Smoke on the Water.  (Dont bother looking that up.)

Have you dripped lye from wood ashes?  What did you use it for?  How did you test the strength?  Let us know in the comments section below!

Historical Lye Making Part 1

One thing thats 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 isnt to say that lye isnt 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, lets 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 whats available commercially) makes it easier to recycle the soap as part of your graywater scheme since you dont have to worry about sodium buildup.  (Plants need more potassium than sodium.)   In addition to soapmaking, youll 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 wasnt 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 havent 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 its more likely that softwoods dont 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 dont 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 shouldnt 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 someones 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.  Dont 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.  (Dont 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, well 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!

Wood Ash Leavening Chemistry An Extraction of Historical Accounts

Leigh over at 5 Acres and a Dream recently did a fascinating series of blog posts on producing leavening from wood ashes (Part 1, Part 2, and Part 3).  The high-level overview is that wood ashes contain potassium carbonate, which can be extracted and used as leavening for quick breads, biscuits, etc., similar to how baking soda is used.

Leigh made some pretty tasty-and-leavened-looking biscuits with her extracted carbonate (and with straight wood ash), but noted that they didnt rise quite as well as the control biscuit (which had baking soda).  There were also a few unanswered questions on the chemistry involved, so we wanted to follow our nerdy instincts and dive into the nitty gritty of whats happening at the molecular level.

First issue: what is actually being extracted from the wood ashes?  Carbonates, we suspect, but is that it?  In our minds, theres a controversy, since the process of extracting carbonates for leavening sounds an awful lot like the process of extracting lye (potassium hydroxide, KOH, in this case) for soap making.  Were especially keen on figuring this out because if both hydroxides and carbonates are present, it will change our biscuit recipe (specifically, well have to add more acid to get the leavening effect). Lets compare some descriptions.

 The very cool Caveman Chemistry website says that the major components of wood ashes are potassium and sodium carbonates, but says this of the extract:

"It contains all of the soluble materials which were present in the the ashes to begin with. This could include sodium and potassium chlorides, sulfates, hydroxides, and carbonates."

So, it sounds like both carbonates and hydroxides could be present.  Another account of potash and pearlash production from 1866 is generally consistent with that (despite a distinct lack of cavemen in 19th century North America), but doesnt mention hydroxides:

"Carbonate of potash is generally obtained from wood ashes...the soluble constituents of the ashes are the carbonate, sulphate, phosphate, and silicate of potash and chlorides of potassium and sodium.  The insoluble constituents are carbonate and subphosphate of lime, alumina, silica, the oxide of iron and manganese, and a dark carbonaceous matter."

That same account also describes the process for preparing the potash and pearlash:

"In America, the ashes are lixiviated [extracted] in barrels with lime, and the solution evaporated in large iron pots or kettles, until the mass has become a black color and the consistency of brown sugar.  In this state it is called, by American manufacturers, black salts.  ... To make the substance called pearlash, the mass called black salts...is transferred from the kettle to a large oven-shaped furnace, constructed so that the flame is made to play over the alkaline mass. ... The ignition is in this way continued until the combustible impurities are burnt out, and the mass, from being black, becomes dirty bluish-white, having somewhat of a pearly lustre, whence the name pearlash. The coloring matter is probably in this case manganate of potash."

In a process flow diagram, it would look something like this:

Other soluble minerals (OSM) seemed like a better acronym than Minerals of Unusual Solubility (MOUS). (Warning: obscure pop culture reference.)  You can buy pure potassium carbonate these days, and its bright white.  To visualize the color of pearl ash, think of this color, but very faint.

So, no mention of potassium hydroxide in the old-time production, but that might be because of the production method.  The CO₂ in the combustion gases that are passing over the black salts reacts with KOH to make KHCO₃ (or to make H₂O and K₂CO₃); any KHCO₃ produced decomposes to K₂CO₃ in the heat.  So basically, if hydroxides are extracted into the ash water, they dont make it into the pearlash.

But, compare the process of making ash water for leavening with any of several similar descriptions of the process for preparing lye for making soap.  For example, this one:

"Traditionally, one uses an old wooden barrel or lye hopper for this, even hollow treetrunks in some areas. ... In the bottom, put a filter made from a couple of inch depth of twigs, and the same again of straw or hay. This helps ensure the lye comes off moderately clear. Stand the lye barrel up high enough to get a container underneath...and fill it up with those ashes. Add water. ... Leave it all overnight...[then] let the lye run out into your container."

Other descriptions call for adding lime (or slaked lime), which we noted increases the hydroxide yield by converting carbonates to hydroxides by the following reaction:

 Ca(OH)₂ + K₂CO₃ = 2 KOH+ CaCO₃

There is also a journal article in the peer-reviewed literature, which claims the ratio of hydroxides to carbonates in their crude ash extracts is 92-to-8, and more anecdotal observations that carbonates dont work very well for making soap (but ash water does) and that crude ash extract by itself doesnt do much leavening.  Therefore, it seems very likely that the crude ash water extract contains an appreciable amount of hydroxides along with the carbonates.

So theres the theory--probably both carbonates and hydroxides are present in the ash water.  Fortunately, we dont have to just sit around, dealing in hypotheticals.  We can experimentally measure the amounts of carbonate and hydroxide in the ash water through the magic of titration. (If youve suffered through an analytical chemistry class in college, we hope you didnt just throw up in your mouth a little bit.)

O ash water, what mysteries containest thou for us to unravel by the labor of titration?

Well give you a few days to stew over that and hit you in a few days with a chemistry-dense post interspersed with colorful pictures.