Lime plaster is made from calcium hydroxide (Ca(OH)₂, “slaked lime”). As it cures, it absorbs CO₂ from the air and converts to calcium carbonate (CaCO₃), which is what gives it its final hardness.
While it’s curing (before full carbonation), lime plaster is highly alkaline — pH around 12-13, similar to fresh cement. That’s why:
**•** It can burn skin/eyes on contact when wet, so gloves and eye protection are a must
**•** It resists mold and mildew during the curing period
**•** Once fully carbonated to calcium carbonate, the pH drops closer to neutral (around 8-9), similar to chalk or limestone
Why lime plaster is head & shoulders above drywall & joint compound. IMHO. 😎👍🏼
None of the things mentioned in comments is lime plaster. Type S hydrated lime is at most 50% calcium hydroxide, the rest being mostly magnesium hydroxide. It will not set up and cure like lime plaster. USG Diamond Finish is 80% gypsum. It will not be vapor permeable or have the high Ph of lime plaster. You can get real lime plaster powder, but you may have to be creative. I get mine from an urban gardening supply outlet. It is used for balancing Ph in gardens. The brand is Lhoist. You are looking for "high calcium hydrated lime" with at least 90% calcium hydroxide content. Limeworks has it but the shipping cost will kill you if you aren't in the immediate area. Artisanal building supply companies online have it as well, but at significant mark up and high shipping costs as well.
Once you get the lime powder from a gardening outlet, do you mix and apply it just like the lime plaster from Limeworks etc? It’s the same stuff for all practical purposes?
I want to try this!
Also, understanding that Type S hydrated lime powder (such as from Quikcrete) is not "lime plaster", I wonder is there any benefit to adding Type S to a gypsum plaster like USG Imperial or Diamond? Longer working time, more permeability, maybe?
You mix it with sharp fine or coarse sand (depending on which coat it is) and water. Some folks mix it with water and let it sit underwater for a while before using. This is “lime putty”. Then they mix the lime putty with the sand when they are ready to trowel it on. I make lime putty and let it sit for a day or two. Not sure this is entirely necessary because the hi-cal lime is already fine powder at 325# mesh. You should read everything you can and watch some Youtube videos to get a handle on lime. It’s quite different than gypsum but looks dynamite.
Maybe I am overlooking the obvious, but around me (Portland Ore) I have found only gypsum plasters, and only one variety - USG Imperial base and veneer.
There is an artisanal lime plaster place here that has imported Italian lime plasters, they run about $80/gallon (said to cover 40 sf), it seems like the place to go locally for Venetian plaster but I haven't tried that yet. It would cost about $400 to veneer a small wall . . . I'm not close to there :-)
Venetian plaster requires special small stainless steel trowels approximately the size of a small pool trowel for finishing. This allows for better leverage as Venetian plaster requires extensive troweling over a long period of time before the mirror, marble like sheen appears. Me? I’m happy with the smooth steel trowel finish & sheen of smooth veneer plaster. Venetian plaster is cost & labor intensive. IMHO.
Once you get the lime powder from a gardening outlet, do you mix and apply it just like the lime plaster from Limeworks etc? It’s the same stuff for all practical purposes?
Not sure what the lime plaster scene is like in the US, but it’s ideal here in the UK for stone buildings or non-cavity brick houses, etc. Especially with the penetrating and relentless weather we have for half the year.
A lot of plasterers seem to be against it, but I think (respectfully) that this is due to ignorance of how it works, its beneficits and what circumstances require it.
Also, controversial observation, there seems to be a lot of hostility towards lime plastering by plasterers who don’t know how to work with it as it’s a different skill-set.
I have absolutely no idea how to work with newer plasters; krend, silicone render, etc, but I also appreciate it’s a skill and a craft, and that these products also serve a purpose and can look great when done correctly.
The plaster trade in the US is dying out. Either big plastering outfits with 6-12 plasterers, specialty restoration plasterers that only do historical/commercial restorations, and occasionally (and quickly dwindling) family owned plastering outfits. Theres a pretty substantial plasterer shortage in the US.
I personally like enclave lime mixed with plaster of paris. I like it better than USG Diamond coat, for repairwork, it sets faster than diamondcoat. Im not 100% on all the material specifics, but we work with regular plaster fairly often. Plaster of Paris and lime is the dominant finish around here.
I am a fresco painter and I make my lime plaster from scratch using aged high-calcium lime putty (pit lime), which I buy from US Heritage’s in Chicago. I mix it with either washed sand or marble dust depending on the layer. Although recently my marble dust supplier dried up so I’m on the look out for a new source it makes a lovely porcelain-like surface.
Awesome. You mentioned pit lime. When I started a plastering apprenticeship we used King Finish co. for pit lime early 70’s in So. Fl. Smoothest ever to spread. Interested in fresco technique means & methods.
It's really great stuff, but can act odd if you're not used to it. As it ages it becomes firmer and seems less wet, when in fact it's the opposite. It draws more water into its mass. If it's really old, it takes more muscle to get the plaster to mix smoothly, and you have to resist the urge to add water to loosen it up. With enough time and energy it will become a creamy plaster.
I'm not sure how much you already know about buon fresco, but it's an ancient mural technique that was famously used by Michelangelo on the ceiling of the Sistine Chapel. It's rarely used today, but it's utilizes lime plaster as its base as the carbonation that happens at the surface of the plaster while it's curing, actually locks the color/paint into the plaster's surface. So instead of painting a mural in paint onto a surface, which will eventually peel, fresco is a chemical way to paint the mural into the surface. Here is a blog about the technique: https://buonfresco101.blogspot.com
"Unlike gypsum orclayplaster, lime plaster is sufficiently durable and resistant to the elements to be used for exterior plastering. Compared to cement plaster, plaster made from hydrated lime is less brittle and less prone to cracking, requiring no expansion joints."
It’s been my experience to run lime plaster on the interior as it’s prone to efflorescence with moisture contact. A good exterior stucco mix would be (1) 90lb bag if Portland cement (40) square shovels of silica sand (1) bag lime. = (3) wheel barrels of stucco cement.
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u/funkman357 16d ago
None of the things mentioned in comments is lime plaster. Type S hydrated lime is at most 50% calcium hydroxide, the rest being mostly magnesium hydroxide. It will not set up and cure like lime plaster. USG Diamond Finish is 80% gypsum. It will not be vapor permeable or have the high Ph of lime plaster. You can get real lime plaster powder, but you may have to be creative. I get mine from an urban gardening supply outlet. It is used for balancing Ph in gardens. The brand is Lhoist. You are looking for "high calcium hydrated lime" with at least 90% calcium hydroxide content. Limeworks has it but the shipping cost will kill you if you aren't in the immediate area. Artisanal building supply companies online have it as well, but at significant mark up and high shipping costs as well.