Basic information and formulas for making fresco plasters

The specific plaster used for buon fresco is lime based. It is beautiful in its simplicity and has been a material for construction for thousands of years. The raw ingredients include two basic items, lime putty and aggregate (sand or crushed marble), but occasionally small amounts of distilled water can be used to increase workability.  Lime plaster should be mixed completely so that no white pockets of lime putty remain. A cement mixing drill can help a great deal when mixing, however small jobs may be mixed by hand with some physical effort.
One measure of lime putty to two measures of river sand

Partially mixed 

Once thoroughly mixed, the plaster is a consistent color and value

All plaster mixes must use stable proportions. Too little lime putty makes the mix dry and crumbly. The aggregate is under bound and doesn't mix well. It refuses to stick to the trowel or wall. In contrast, too much putty makes the mix unstable as there is not enough aggregate to stop the lime from shrinking leading to cracks and/or delamination (see the aggregate page for a description of its function in plaster). 

Similarly, when water is added to plaster to increase workability (the ability to be easily spread) it's important that it happens only after the aggregate and putty mix has had sufficient time to bond, which is a day in a sealed bucket (see the aggregate page on the importance of dryness). A sprinkle or two of water can make the mixture less stiff. It becomes more elastic and makes the material both easier to spread and more sticky. Problems with plaster's lack of stickiness can be caused by a mix being too dry or wet. In both cases the plaster may fall off the trowel or wall. 

Notice in the picture above how the intonaco plaster sticks to the trowel even while upside down. If the plaster is too wet it will slide off the trowel; if it's too dry it will be too crumbly to adhere as well.

Although, the number of layers and their thickness may vary depending on situational needs, a general structural principle should be observed. The wall's layers should progress from thick to thin, from coarse to fine aggregate, and from lean to fat in regards to the ratio between aggregate and lime putty. 

Common Plaster Layers in Buon Fresco: 

The "scratch" coat: This first coat for wall applications is typically 2.5 parts coarse aggregate (#14 mesh) to 1 part lime putty and when appropriate 1 part or less of Portland Cement. White cement is preferable but not necessary. Gray Portland Cement works too. Initially, this plaster will be more stiff and may even lock up while mixing because the very fine cement powder has greater surface area. You may need to add small amounts of water to get the mix to a manageable state. It should have body but not be so thick that it resists attempts to spread. However, it should be dry enough to not slump.

It's important to note that cement is cured using water rather than carbon dioxide so it cannot be sealed in a bucket and used the next day. I mix only what I can apply in 20 min and stir the mixture occasionally to keep it from setting up. Also, a cement mix will require additional moisture to cure properly. Lack of water during the curing process will result in cracking. Mist or spray water onto the layer every few hours for two or three days after it's application. That being said, I have read that some practitioners employ a controlled cracking of this layer.

The "brown" coats: Subsequent coats are oftentimes referred to as leveling or equalizing coats, as they are evened using a screed and/or wooden float, and may number between one and three. They are the intermediate layers between the scratch coat and last painting coat.  Typically they are made by mixing 2 parts of aggregate (#20 mesh) to 1 part of lime putty. Brown coat plaster can be made in advance by a couple days and stored in an airtight bucket. It must be mixed up again upon use and may be sprinkled with water to increase handing. As mentioned earlier, the goal is to add only enough water to make the plaster more manageable and increase its ability to stick to the trowel and wall. Wet plaster should be elastic and sticky (not runny) and not break or crack easily when folded onto itself as if you were folding an omlette.

The "sand" coat: This penultimate coat is followed immediately by the painting coat, which is applied the same day! It is identical in mixture to the brown coats and is oftentimes referred to as such. Once the sand coat is firm enough to resist pressure from a finger (about an hour), the painting coat can be applied. Stephen Pope Dimitroff suggests that the proper dryness/firmness of the sand coat can also be tested with the back of your hand. If sand granules or moisture stick to your hand, more time may be needed for curing. 

The painting"Intonaco" coat: The final coat is the painting surface and can be made by mixing 1 part of fine aggregate (#30 mesh) to .75 parts lime putty. Some frescoists use 1 part aggregate to 1 part lime, however I prefer something less rich. I find a little less putty helps make the plaster more stable.  I have used 1:1 in the past and found the lime crust can at times begin to pull off the painting after 5-6 hours of working, leaving a bald spot. I imagine this is due to the surface being too fat. Thanks to some advice from iLia Annosov at the Fresco School, the 1 to .75 mixture has proven to work splendidly.  As this is the painting surface, many artists prefer to use crushed white marble dust as the aggregate, which gives a whiter surface and also lessens the amount of color change between wet and dry coats allowing for easier color matching from one day's work to the next. This is in contrast to sand, which is significantly darker while wet when compared to its dry state.  

Plaster made with river sand is brown

Plaster made with white marble dust is white
This is commonly used for the painting layer
known as the "Intonaco"

Finally, great care should be used when mixing marble dust and lime putty as both are white.  The whiteness of the marble makes lime pockets hard to detect. For this reason it is advisable to double the mixing time to ensure that no lime pockets remain unmixed. Poor mixes may cause "hot spots", uneven drying, and irregularity when painting. Severe cases may also lead to structural instability.

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