18_Slaking -- a tale of two limes

Time to slake more lime. We ran out of lime putty (which we slaked last March, from the hydrated lime that we bought from the tortilla factory in Nogales, Mexico).

This time I wanted to slake quicklime.

Ultimately I bought 2 kinds of lime in Tucson -- a sack of quicklime from Adchemco, and a sack of hydrated lime from Mi Casita tortilla factory. The 2 limes could not have reacted more differently in water, like the lion and the lamb. The quicklime gurgled, spit, sputtered and boiled after sitting in water. The calmer hydrated lime took its time absorbing water, behaving almost finicky, pretending to be more and more hydrophobic as the bucket filled. However the smaller sack of violent quicklime yielded twice as much lime putty as the larger sack of docile hydrated lime -- 6 buckets versus 3 buckets:


To slake lime, one merely adds dry lime to water, then waits months. We used the cheaper "reverse osmosis" drinking water from Circle K ($1 to fill a 5 gallon jug), rather than the more expensive distilled water in gallon jugs. It took 30 gallons of water to slake the two above sacks of lime. Hopefully there are no impurities in our water that would promote cracking in the final fresco:




First, I should emphasize the basic goal of fresco -- to convert natural limestone into a limestone painting. Fundamentally, it is mostly a shapeshifting challenge, that of distorting a lump or pile of limestone rocks into a thin, even, flat limestone surface. Somehow we need to manipulate the elusive elasticity of limestone, and add a few colored impurities on top. The pigment impurities will become one with the limestone at the end of the long shapeshifting process. Again, in buon fresco, the colors are part of the final limestone, not just merely painted on the surface, like with fresco secco.

The actual shapeshifting procedure is fairly convoluted. First one burns limestone (calcium carbonate -- CaCO3) to create quicklime (calcium oxide -- CaO). Then one adds water to the quicklime to create hydrated lime (calcium hydrate -- Ca(OH)3). Apparently if one only adds about 25% water, the calcium hydrate remains dry, in a form which you can buy commercially in a sack, like the larger bag of powdered hydrated lime we bought from the tortilla factory. However we still need to add more water to produce a lime putty. Then this lime putty has to slake for at least 2 months to absorb the water -- better if it slakes 2 years or longer -- before one can mix it with sand to make fresco plaster. Once the plaster is troweled onto a wall or panel, and its water evaporates, it starts absorbing CO2 from the air; and over the next year or so, slowly reverts back to limestone (calcium carbonate). This article elaborates on the differences between hydrated lime, quicklime, and lime putty.

However this link explains the process more succinctly, which I have illustrated as a cycle in the diagram below:


Buying good fresco lime to slake is not so easy. We needed to find a "high calcium" lime -- like the quicklime or hydrated lime from Mississippi Lime. Commercial limes made from dolomite have a high magnesium (MgO) content, which helps it retain water, but promotes cracking in the final fresco painting. Professional plasterers, however, apparently prefer the high magnesium content, because I hear that it makes the lime plaster easier to spread (note, many professional floater trowels are made of magnesium). Thus, we cannot use the cheap "Type S Lime" for plasterers, sold at Home Depot, for anything but practice fresco, if we intend to produce a quality, unblemished buon fresco painting (thanks to The Fresco School for pointing this out in a comment at the end of an earlier post).

• Additionally, there are various "types" of hydrated lime, like "type S" and "type N," to further confuse the choices. Moreover, the chemists carry different grades of lime -- listed from highest to lowest quality: ACS, Reagent, USP, NF, Lab, Pure, Technical. All we want is the cheapest lime that will not crack while drying. I decided that "food grade" lime should be good enough, and discovered that Mississippi Lime is the only company in the US making "food grade" lime.

• Note also: fresco lime is calcium carbonate. Plaster of Paris is calcium sulphate -- which is not to be added to the fresco lime. And our bones are made of calcium phosphate.

Safety materials needed to slake lime:


Quicklime is extra hazardous:


In March when I slaked hydrated lime, I used a cheap mask, but the dust still irritated my lungs and gave me a lingering cough. This time I used a better mask -- the 3M 8511, Sanding Painted Surfaces Respirator (N95 NIOSH approved) -- and spared my lungs. It even protected me well from the more caustic quicklime dust:



The neoprene gloves made a huge difference also, protecting the flesh on my hands:



Briefly, I did handle the lime without the neoprene gloves, and suffered for it. In a delayed assault, the quicklime dried out my skin, and dug a wound into my finger. For days afterwards, my scaly hands hurt, though I did recover fully in about a week:





Shown are the materials needed to slake lime. A garbage bin to store the sacks of lime after I opened them. Gallons of purified water. Safety masks and gloves. 3 mil large garbage bags, 5 gallon storage buckets with lids, and rubber bands. A flour sifter and white buckets (and a plastic scoop, not shown):


I used a 5 gallon jug of purified water, making several trips to the Circle K, to refill it during the slaking process:



Scooping lime into the flour sifter:


Sifting lime into water. The idea is to avoid chunks of dry lime, which might resist absorbing water at their cores:


When we sifted too fast, both types of lime would resist the water, and coagulate into a dry "football" floating in the middle of the bucket. A mosaic pattern, resembling a dried lake bed in the desert, would then form on the exposed top of that "football," as the water intruded into the mass. However, it took too long for the water action to sink the dry football. Thus inspired by the The Fresco School's excellent YouTube video on slaking lime, and impatience, we decided to stir the lime slurry with a non-reactive wooden stick, to speed up the mixing process. When we noticed small splinters in the lime, we switched to a plastic rod for stirring:


The hydrated lime did not misbehave when we mixed it with water. The bucket did not even warm up.

However, after dumping quicklime in water, the vat would gurgle and bubble up, but not right away. Rather, the first clue of discontent came when I touched the surface of the water, and it sparked back. Much later, after the wet lime accumulated somewhat at the bottom, the plastic bucket itself heated up in a delayed reaction, until it was too hot to touch the outside. I would sift in more lime, and stir the slurry up with the plastic scoop, to dissipate the heat. However, eventually the heat even overcame my neoprene gloves, and I had to leave the bucket alone to cool off a bit.

Then I would sift quicklime into a second bucket, until it also became too hot to work with. Had I a third bucket handy, I could have slaked that sack of quicklime even faster:


When the white buckets filled up and eventually cooled off, I pushed a pair of large, heavy duty (3 mil) garbage bags, into each of a few 5 gallon orange buckets. Then I poured the thick, beautiful, very white lime cream into the inner bags. I then added extra water, before tying each bag closed with rubber bands. Hopefully between the double-bagging and excess water, the lime will keep from drying out in Arizona weather:

• Note: I washed all the lime tools in a separate plastic bucket, and dumped the water in a larger trash can. I did not pour any lime or lime water down the drain, realizing that it would ruin the plumbing.

We made 6 buckets of lime putty from the quicklime, which still needs to slake until at least next March before we can use it. Perhaps the immediate problem is keeping the lime putty from freezing during the Tucson winter:




We made 3 buckets of lime putty from the hydrated lime, which must also not freeze, and needs to slake until next March:




Next time we might slake lime in a clean 55 gallon plastic barrel, to make the ordeal go faster. Moreover, a single barrel would be easier to check periodically to add water, before the lime putty threatened to dry up. We bought a leftover plastic 55 gallon barrel, but did not use it because there was some unknown residue that might have polluted the lime putty:


Of course, traditional lime pits would probably be best. Scroll down through Daan Michael Hoekstra's blog to see the slaking lime pits in Alamos, Sonora, Mexico.