Ancient installation methods
The 8 1/2” by 19 1/2” mosaic called the Standard of Ur, which dates to between 2600 and 2400 BC, was discovered in the Middle East in a burial chamber in 1927, its wooden frame and asphalt glue decayed by the passage of over 4,000 years. Now reconstructed as a hollow wooden box, it is inlaid with lapis lazuli, carved shell fragments and red limestone.
Of particular interest to modern tile installers who often argue among each other over the merits of glue versus mortar adhesives, attachment of the mosaic bits (both the original and the reconstructed version) is done with bitumen. Today, the word bitumen applies to a wide variety of hard and soft petroleum-based materials. In 2600 B.C., bitumen referred to petroleum tars obtained from naturally occurring seeps.
Although asphalt materials were used to construct small, decorative mosaics thousands of years ago, most functional mosaics were made with mortar bases and adhesives similar to materials used today. Soft mortars were made with fine sand, crushed shells, pottery shards and lime. Hard mortars were mixtures of many of those materials as well as gravel, marble dust and pozzolano – what we today call volcanic ash.
To make colored grout, ancient mosaic craftsmen crushed marble of a desired color into a fine dust and mixed this with pozzolano. Construction of functional mosaics was robust enough that even after the passage of thousands of years, many surviving mosaic installations still retain their sparkle and color (Photos 1 and 2).
Ancient mosaic compositions were constructed using two basic methods: direct and indirect. The directmethod was used primarily for small panels, where mosaic bits were adhered directly to the mortar, stone or wood base. Although this method could be used for mosaic panels of any size, it was generally limited to smaller works such as the Standard of Ur. Some mosaic artists applied a thick layer of adhesive mortar so that once a small area was covered, a flat board and hammer were used to flatten the surface and force the fine mortar up into the gaps between the mosaic bits – called tesserae.
The indirectmethod, on the other hand, was used primarily to construct large areas. There were many different methods. One of the oldest employed soft clay, fabric and water-soluble glue. First, soft clay was prepared by rolling it into a layer about 3/4” thick. Next, tesserae were pressed partially into the clay and the surface of the bits was tapped into a smooth plane with a flat board and gentle taps of a hammer.
With this done, a section of cloth, soaked in a water-soluble glue, was draped over the bits and allowed to dry. The resulting sheet was pulled away from the clay and the excess cloth was trimmed away. Once all the tesserae making up an entire panel were thus converted, the sheets could then be installed significantly faster and more efficiently than if tesserae were installed individually.
The present day
Although some of the materials and methods of producing tesserae are different now, the basic process of preparing, installing and finishing mosaic panels has remained unchanged for thousands of years. Replacing fabric attached to the surface of a grouping of mosaic bits, a sheet of paper affixed with water-soluble glue is now used (Photo 3).
The paper serves two functions: to secure the mosaic bits, and to act as a stop for grout, which is troweled onto bits before the sheet is installed. Once the adhesive mortar has begun to setup, the paper is moistened and removed. Timing for this part of the installation is critical – remove the paper too soon and the bits might shift. Wait too long and repositioning any errant bits is difficult. Even worse, grout that has set up too hard may require such vigorous cleaning that many bits may be dislodged, creating a real mess and additional work.
An alternative to paper face mounting is back mounting (Photo 4),which can be done with fiber mesh, perforated paper sheets or flexible dots. Both fiber mesh and perforated paper sheets are glued to the backs of the bits, which are held in place in a grid to keep the bits aligned during the process. With dot mounting, the bits are arranged in a grid and then all the bits are lifted with suction cups. During this operation, another machine deposits carefully aligned adhesive dots on a non-stick tray. Once the dots are deposited, the mosaic bits are lowered onto the grid of dots and the tray is moved to allow the dots to harden.
The automated process of installing face- and back-mounted sheets is largely used by mass-production mosaic manufacturers. Another method for mounting mosaic tiles, most often used by art tile studios, is to cover an arrangement of mosaic bits with a clear, self-adhering plastic sheet (Photo 5).
Comparing mounting methods
For each type of mount, there are benefits and drawbacks. Although paper face-mountedmosaic sheets can be installed first and grouted after the bits have hardened off, the preferred – and traditional – installation method is to trowel and pack grout on the back of the sheet, and immediately install the sheet over a bed of mortar adhesive troweled onto the setting bed surface. This method requires considerable skill, with the drawback that the mosaic bits are obscured by the paper sheet, so repositioning can only be done once the paper has been removed. Timing is critical.
Paper mountings occupy a rather large percentage of the backs of the individual mosaic bits. As such, the paper mesh reduces bond strength since it reduces adhesive contact between the adhesive and the mosaic bit. Paper and the glue (if water-soluble) that bonds the paper mesh to bits can dissolve when installed in wet areas, and paper can contribute to mold growth.
The strands of fiber mesh backingare relatively thin and do not interfere too much with the adhesive bond, but the glue holding the mesh to the backs of the bits may. Additionally, if the glue is water-soluble and the mosaic sheet is installed in a wet area, the glue may dissolve, and given the small size of the mosaic bits, this can significantly reduce bond strength.
Dots only marginally reduce adhesive bond, and since they are not water soluble, are not a problem in wet areas. But dots can sometimes affect the uniformity of some grout colors since the dot material does extend into the joints between bits.
Clear plastic mounting sheets,since they cover the face of the mosaic bits, affect neither the adhesive bond nor the grout. In addition, since the sheets are transparent, out-of-position mosaic bits can be spotted easily, and by cutting through the sheet, any errant bits can be repositioned without affecting the remainder of the bits in the sheet. Another advantage is that the sheets prevent rapid evaporation of the moisture in the adhesive mortar, resulting in higher bond strength.
Because most mosaic tiles, tesserae, and bits are relatively thin, clogging the grout joints can be a problem when the adhesive mortar is spread with a notch trowel. Notch trowels are the only practical tool that can lay down an even layer of mortar, but ridges produced by the trowel will create a cleaning nightmare if too much mortar oozes up into the joints (Photo 6).
This potential problem can be avoided by using a three-step application method. First, the flat side of the trowel should be used to key the mortar into the setting bed. Next, the notch side should be used to comb out a uniform layer. Finally, the flat side is used again, this time to gently flatten the adhesive ridges (Photo 7). A uniform amount of mortar has been spread on the setting bed in Photo 8, but the flattened area is lower and consequently, mortar does not extend more than 1/3 the way up into the grout joints.
Being successful at mosaic tile installation takes finesse, patience and practice. It requires hard work, but the results can be visually stunning. If you are considering adding this skill to your toolbox, make sure to take the time to learn it well.