3 Reasons Why Wood is a Bad Mosaic Backer

Wood is easy to find, it’s cheap (sometimes free), easy to cut into many shapes, it’s rigid, easy enough to install hanging systems, etc. So, what’s not to love about wood?

It has 3 properties that are destructive to mosaics. But, before we get to them let’s think a little about the history of mosaic art and why the destructive properties of wood should be avoided.

Our first known mosaics are from the second half of the third millennium BC – about 4,500 years. They adorned a temple with patterns made by the tesserae. The ancient mosaics we often see today are from the Hellenistic period (starting about 2,500 years ago) and the Roman Empire (starting about 2,200 years ago). They often depicted mythology, nature and early Christianity with colored stones and were installed on the floors and walls of homes and places of worship. Having had the wonderful experience of seeing and even walking on these ancient works of art, I know the awe. One big take-away is, the ancient mosaicists made mosaics to last.

It seems that if modern mosaics are to honor their heritage, they too should be made to last for millennia. The tesserae used in modern mosaics like stone, ceramics and glass will last indefinitely, but wood backers (substrates) will not last nor honor the heritage of mosaics. The reason - the 3 destructive properties. They are:

1 - Moisture transfer. So that it can grow, the structure of a tree is made to transfer moisture. When it is cut and milled, that property remains. So, wood used as a backer for a mosaic absorbs and releases water from the air (humidity) and other sources. As it absorbs water it expands and contracts as it releases the moisture. Eventually, this expansion and contraction breaks down the adhesion and the grout. That facilitates greater moisture transfer and ultimately the release of tesserae from the backer.

2 - Thermal conductivity. Heat causes some materials to expand more than others. Some have almost no response to heat, like Styrofoam and fiberglass, until the heat reaches its melting point. When heat dissipates, the material contracts. Similar to moisture transfer, these cycles of expansion and contraction break down the adhesion and the grout and ultimately result in the release of the tesserae from the backer. 

3 - Rot. Because wood is organic, it is susceptible to attack by fungi which digest the fibers that give wood its strength. Some fungi work when the wood is wet, others when it is dry. Thus, the terms wet rot and dry rot. Whichever type of rot, the result is decomposition of the backer that the tesserae have been attached to, and ultimately the release of the tesserae from the backer.

Even if your mosaics are indoors, these 3 destructive properties of wood still apply. They may last longer, but the wood will inevitably rot. If you will be making mosaics from tesserae that will decompose, then you might as well use a wood backer. But, if you want your mosaics to be enjoyed through the ages, put them on a backer that will last as long as the tesserae. Skeewbackers, Clearbacks, most foam-core wall boards, and concrete substrates will last as long as your tesserae.

If you would like to learn more about mosaic backers get our free book, The Book on Mosaic Backers.

And, in case you are curious about the construction of ancient mosaics in the Roman Empire, they put the tesserae on concrete and used mortar as an adhesive (they invented both the 3^rd century BC). Today, thinset is even better than mortar and foam-core substrates are much lighter and easier to work with than concrete.