Luxury & Lime - Advanced Application Course

Slaked Lime Production

Production of Seasoned Slaked Lime

Lime is the most “antique” binder  after gypsum and used until the industrialization of building. “Antique” like the Egyptians, Greek and Romans. The architects of Renaissance, among others also Andrea Palladio, describe it very well.

Slaked lime - Antique finishes based on lime 1

Antique finishes based on lime 2

..."You know it’s called Affresco because one paints on plaster, made of fresh lime and sand,  that is still wet and dampish, not dry and hard.  A great capacity and rapidity is needed while spreading the colour and creating forms because the plaster absorbs rapidly. The spread colours dry in a few hours and become hard which forms a  crust of colour plaster. It’s necessary to be very determinate when painting. Only  few and firm strokes are sufficient, because if passing more times to correct the colour, the plaster is unable to absorb and once dry the layer of plaster could exfoliate and fall off."...

Fresco 1

Taken from : “Impossible interview” – Piero de Fina dressed the role of Giovan Battista Tiepolo – Artist and art historian. Lime is obtained by heating limestone, composed mostly of carbonate of lime, at high temperatures. The limestone are reduced in pieces (if not already in pebbles), put into the furnace and calcine at 700-900°C to become quicklime (oxide lime CaO).

River pebbes - Slaked lime

Calcination of Limestone
Calcination of limestone

Quicklime is a porous mass, so greedy of water to hydrate spontaneously if exposed to air. According to the quantity of impurity its colour vary from white to yellowish. For years two types of furnaces were used for the calcination of limestone: Batch furnace and continuous kiln. Today rotary kilns are used.

slaked lime furnace

De Toni Family slaked lime furnace

Image of the furnace owned by the De Toni Family, dated 1930.

Slake of Quicklime

The slake of quicklime and its reaction with water:
Slake of quicklime
The reaction develops a strong heat and the volume increases of approximately 10%. Due to the high temperature, that can be produced istantly (over 150°C), the lumps swell, crumble and the water turns into vapour. Therefore, it's necessary to add water a little at a time and mix the paste with care.

Slaked Lime paste

Types of Lime

Lime Hydrate (Superfine Lime)
Obtained by using a quantity of water almost equal to the quantity of lime.
The hydrate is a soft powder and finely separated. Sold in paper bags, like cement, and it must be kept in a well dry place. It’s less sensible to frost than grassello.
Lime Grassello. Obtained by using a quantity of water higher than the quantity of lime (approximate ratio 3:1). The Grassello becomes a plastic mass and oily at touch. In the past, rarely also today, the lime was slaked in holes in the ground where it was put under water for months (aging).  It’s important to know that an insufficient slaking of limestone lumps, causes serious problems for mortar. The lumps slake later after use, increasing their volume and therefore creating swells and crumbles. Nowadays, grassello is sold packed in plastic bags, ready to use.

Setting and hardening of Lime

The main characteristic of lime is to compose a product that, when mixed with charges, results easy to work and mold until damp. Once dry, the product is quite water-resistant and has good mechanic properties as well as stability in time.
This characteristic is based on a chemical reaction of slaked lime and carbon dioxide :
Chemical reaction of slaked lime and carbon dioxide
While the mixture is drying, the water evaporates and forms carbonate of lime.
This is called CARBONATION: lime returns to the identical composition of the limestone but with very smaller crystals than in origin. The reaction of carbonation is very slow due to the low concentration of carbon dioxide in the air therefore the complete transformation of lime hydrate in carbonate needs even years. This is called “air” lime since it’s necessary the contact with air for setting. The evaporation of water causes a shrinkage of volume.
For this reason slaked lime is never used alone but always with an inert charge to avoid breaking.

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