Aggregate

Aggregates used in lime plaster include particles of sand or crushed rock such as limestone or marble. As pure lime putty dries it contracts and eventually cracks. When bonded to an aggregate like sand or marble dust shrinkage is limited by the amount of rock particles that can fit in a particular volume. As an example, imagine an open hand full of loose rocks. The hand is the lime and the rocks are the aggregate. As the hand contracts to a fist the rocks are forced closer and closer together until they finally meet and become gridlocked. In essence, aggregates work in a similar manner with plaster. They give plaster a variety of characteristics and when used properly add body and the stability needed to prevent cracking. 

Regardless of whether a plasterer uses sand or marble dust the particles should be angular rather than round as the former creates a tighter interlocking structure. Sand must be from a freshwater source. Ocean sand is not suitable for reasons that it contains salt (an enemy of traditional fresco) and its round shape is less desirable. All aggregates must be washed of impurities that would weaken the bond or dirty the painting surface. The sand must be completely dry and sifted into various sizes.

Instructions on preparing your aggregate for fresco use:



1. Wash:

Put a manageable amount of sand or marble dust into a bucket (1/4 of the bucket).

Using a hose or spigot fill the bucket with water and stir briefly. The aggregate will fall to the bottom while the dirty water and other particles (foam, debris, etc) will float to the top. Carefully pour out as much dirty water as you can while taking care not to loose aggregate to the sink. You can protect your sink with an drain extension or to be really safe do this outdoors with a garden hose. You will repeat this step until the water runs clear from the bucket (you can easily see your hand when pouring the water over it). Once clean, pour out as much water as you can from the bucket. To speed up drying, you may want to tamp the bucket on the ground several times to bring excess water to top for easy pouring.

Note: Dirty sand will look like chocolate milk when mixed with water (see image below). In contrast, dirty marble dust will resemble white milk.

Water that comes into contact with unwashed sand
 becomes dirty as seen above

After several cycles of adding then pouring off the
water becomes visibly clear when poured over your hand. 

 

2. Dry:

Spread the wet aggregate in a thin layer onto a clean tarp. Drying can take place outside in the sun or indoors using a soft fan overnight. 

Aggregate must be completely dry before mixing into plaster. Water forms a liquid barrier around the rock particles that prevents bonding with the lime. Dried aggregate will be noticeably lighter in color and warm to the touch. It should be loose not sticky. 

As the sand dries around the edges, it lightens visibly

3. Sift:
Multi-layer plaster aggregate must be sifted into various particle sizes. 

The darker sand in this picture is wet.
The lighter sand is dry and ready for sifting.

We use a #14 mesh sifter for scratch coat aggregate, a #20 sifter for brown coat(s), and a #30 for the painting layer (the marble intanaco). Tip: Mesh size in the US is determined by the number of squares in a linear inch. The larger the number, the smaller the holes in the mesh.

Store clean, dried, and sifted aggregates in a clean bucket with a lid and label it for easy identification.