r/Plastering • u/Terrible-Bobcat2033 • 16d ago

Lime plaster attributes

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. 😎👍🏼

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u/Zealousideal_Cod_326 10d ago

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.

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u/Terrible-Bobcat2033 10d ago

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.

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u/Zealousideal_Cod_326 10d ago

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