DOUBLE REVERSE METHOD (THE RAVENNA METHOD)
The mosaic is first made on a temporary bed of LIME PUTTY (use hydrated lime - it must be an air-setting lime NOT an hydraulic lime!) or ordinary CLAY.
Before laying the tesserae, you must create a tracing of your cartoon and then reverse it onto to the lime bed. When the tracing paper is lifted, the cartoon outline remains on the lime or clay. To keep the bed soft, sprinkle/spray the surface with water and cover it with a plastic sheet each day or more frequently depending on ambient humidity.
The lime will stay soft for weeks and even months. The lime is spread on a particular type of board made of straw and cement, which absorbs water and slows down the drying process of the binder (particle board may probably be used).
To lift the mosaic from its temporary base, an open-weave fabric (such as several layers of cheese cloth) is glued to the surface of the mosaic. The glue must be water-soluble. The type used by mosaicists in Ravenna is RABBIT SKIN GLUE. This glue comes in pellets because it is very strong and dissolves in water easily. Unused glue can be melted and used again and again.
You can also use a FLOUR PASTE (the kind of glue used by bookbinders), but this type will not keep long, or any type of water-soluble glue (even white glue). The advantage of this method is that you can carefully create a more exact mosaic and make changes to it before placing it into its permanent binder.
After lifting the mosaic from the temporary base, the mosaic is then laid on a bed of a permanent binder made of cement such as:
1 part PORTLAND CEMENT + 1 part LIME + 1 part SAND (river sand without salt!) or marble dust. This mix dries up in about 3 hours. Or:
2 parts PORTLAND CEMENT + 1 part LIME + 2 parts SAND (river sand without salt!) or marble dust. A more resistant mix if the mosaic will be placed outdoor.
DIRECT METHOD
Under this method, you place tesserae, section by section, directly into a bed made of a permanent binder of cement or other adhesive.
You can choose from a wide range of TILE CEMENTS that are commonly found in the building trade such as the "thin set" or “quick-set.” You can try the following products. “MAPEI Ultra/Contact RS,” available at Dal Tile Stores and Lowes Home Improvement. “Marble and Granite Stone Setting Adhesive,” made by Custom Building Products is great for moisture sensitive settings and green and black marble and is available at Home Depots. “Weldbond” is an excellent glue for smaller projects and finishing corrections, dries clear, and is available in the di Mosaico on-line store.
With the direct method, however, you cannot "print " your tracing. The tracing must be performed section by section.
The DIRECT METHOD can also be used to make mosaics by fastening tesserae to a FIBERGLASS MESH. This method works well if you make a small mosaic panel (maximum size approx. 30" x 20") that is then installed elsewhere (wall, floor, etc.). You may stick your tesserae onto the mesh using a thin layer of tile cement or Weldbond. To install the mosaic, a thin layer of tile cement is spread directly onto the receiving surface and then the mosaic "panel" is simply pressed into this bed of binder.
INDIRECT METHOD
In this method, tesserae are fastened face down using water-soluble glue onto a strong paper sheet, which bears the inverted design.
When the design is complete, it can lifted and then pressed into a prepared setting bed of permanent binder. When the binder is set, the paper is dampened and removed.
This method will not tolerate change because once it is set there is little chance of correction. Its primary advantage is that it is a quick method, but it leaves a rather smooth surface, an aspect which is not present in classical pieces.