Many different calcium compounds are called by the same name, Lime. Which is needed for fresco, where is it acquired, and how is it processed?



Use high-calcium lime putty that is at least 94% calcium for fresco construction.

Calcium carbonate is plentiful in nature and can be found as limestone, marble, and even sea shells. When heated at high temperatures it releases carbon dioxide and becomes a new material known as calcium oxide, which is sometimes referred to as quick lime, hot lime, lump lime, and rock lime.

The combination of calcium oxide and water create an exothermic reaction that generates heat and forms a new substance known as calcium hydroxide (hydrated lime and/or pit lime). Calcium hydroxide is an essential material in buon fresco. Check out this Youtube video to see the slaking process: Lime slaking video (Youtube)  

Calcium hydroxide comes in two forms. The first is putty commonly referred to as slaked lime or pit lime, which is made from soaking calcium oxide directly in water. The second form is a hydrated powder that is created in an autoclave. Lime powder is more efficient to ship as it contains far less water than soaked lime putty. It's also less reactive than calcium oxide. Oftentimes, water is added to the lime powder to make a putty that looks similar to the slaked lime putty. As lime putty ages (particularly the slaked lime putty) it becomes more dense and gains qualities such as plasticity, increased water retention, and better carbonation. The older the lime putty, the better!  Many renaissance  fresco artists used lime that was ten years or older.

Once in contact with atmosphere, calcium hydroxide reacts with carbon dioxide in a process called carbonation that reverts the lime back to its original substance--calcium carbonate (limestone). This process forms a crystalline mesh at the surface of the wall. It is the mesh that locks pigment particles into the plaster. This crystal skin is commonly referred to as the lime crust and can be seen by striking a dry plaster wall from the side with a powerful light. The lime crust also appears at the water's surface in the sludge bucket used to clean the trowels.

In short, lime goes through a cycle: Calcium Carbonate + Heat => Calcium Oxide + Water => Calcium Hydroxide + Carbon Dioxide => Calcium Carbonate.

A note about the differences between hydrated lime and pit lime putties:
Hydrated lime comes packaged in paper bags with pin holes. The holes allow atmosphere in and out of the bag to help reduce pressure ruptures as the bags are commonly tossed around. Due to exposure to atmosphere, older bags risk being prematurely converted (partially) to calcium carbonate. More importantly, there has been study on the differences between the two forms at the nano level. The industrial processing (specifically the drying) of hydrated lime powder results in irreversible changes to the lime that detrimentally affects its quality. For more on this topic see the following publication:

Hansen, Eric F., et al. “Lime Putties and Mortars: Insights into Fundamental Properties.” Studies in Conservation, vol. 53, no. 1, 2008, pp. 9–23.,


For those near Bowling Green, KY, Hydrated Lime can be purchased locally from:
Tennessee Farmer's Cooperative (located in Springfield, TN)
phone number: (615) 384-3573 
Cheney's Hydrated Lime $7.50 per 50lbs bag

Regional/National Hydrated Lime Distributors:
Traditionally slaked Pit Lime can be purchased here:
  • The Fresco School (Traditional Slaked Lime Putty and a complete assortment of fresco-making items and materials) 




Making Lime Putty from Hydrated Lime Powder:
Ideally, a fresco painter would use pit lime that has been slaked using traditional methods (see the youtube videos in this blog on slaking calcium oxide). Because of the dangers associated with burning and slaking lime (gasses and heat), we chose to use calcium hydroxide in hydrated powder form. Be sure to use Hi-calcium hydrated lime.

In addition to a mortar mixing drill, students used a scale to measure the weight of 50lbs bags to ensure the same quantity of powder is in each bucket.

lime buon fresco

Outdoor ventilation is important. In addition to using masks, we set up a fan to gently blow powder plumes away from those who were mixing the putty. You should wear long sleeves, pants, and closed-toe shoes to prevent or minimize lime contacting your skin. Improper exposure to lime can cause a number of health issues ranging from dry skin to chemical burns or poisoning. It can even shift your body's PH levels!  


When possible, use freshly made hydrated lime. You can check the shipping date on the back of bag for a clue to its age. The lime was most likely shipped shortly after being manufactured. The younger the shipping date the better! Try to find hydrated lime that is less than 6 months old. You can store unused bags of hydrate in tightly wrapped plastic bags to increase its shelf life.

Steps for mixing the lime slurry (which thickens with time into putty):

1. Add 3 gallons of distilled water to a clean empty bucket.

2. Slowly sprinkle up to 25 lbs (half the bag) of lime into water while gently mixing with the drill bit. When the mixture thickens, add a quarter of a gallon of water to thin it out. When it's all said and done you will have added a half a gallon of distilled water, leaving a quarter for topping off, and a remaining quarter that will go unused.

Weigh the bag of lime as you go to make sure that you have added the proper amount of lime (25 lbs). When finished, the consistency should be similar to a soft milkshake. The mix should cling to drill bit and slowly drip (plop) off. It will also leave a raised trail at the surface (see pic below). Also note that it takes approximately 15 minutes to make one bucket of lime putty in this manner. 


3. To top off the bucket, gently add a quarter gallon of distilled water to top of mixture. This inch or less layer of water will sit on top of putty and protect it from the atmosphere.

4.Date the bucket with a marker, seal it and you are done! Be sure to store it someplace that doesn't freeze.

No comments:

Post a Comment