Technical tips (Issue 2)

ENVIRONMENTAL FACTORS FOR EXTERIOR MOSAICS

When creating mosaics for outside, the final environment should drive the selection of substrate, adhesive, tesserae and grout. 

If your piece is going into a freeze/thaw environment, you should choose materials that can withstand these conditions. In these environments, water droplets expand and contract, which can cause mosaics to fail. Even sealing is not a 100% guarantee against failure. Pieces should also be resealed regularly, but often aren’t. Moderate climates are more forgiving, but correct material choice is still important.

Also, consider if the mosaic will be:

• constantly submerged or in a wet area

• exposed to rainfall or humid conditions

• exposed to constant direct sunlight (UV)

• subject to capillary action and/or negative pressure from soil (for example, from behind a retaining wall), which may compromise your mosaic.

• subject to foot traffic

• installed in an area that’s accessible to people (particularly children) and grind off any sharp edges

• adhered to an external wall (large mosaics). If so, you should consult a structural engineer to make sure the mosaic won’t compromise the wall’s integrity.

Researching and knowing the final environment and using the correct materials will save you so much time, energy and money – especially if it’s a commission piece.

Watch this video by Peter talking about some more considerations to take into account when creating an exterior mosaic:

TIPS FOR CHOOSING THE RIGHT SUBSTRATE FOR EXTERIOR MOSAICS

Avoid using timber for outside projects as it can interact with the environment, causing it to deform and warp. It will also break down over time. Sealing can slow down, but not stop, this process. Generally, MDF and marine ply are only suitable for inside where there’s no moisture. Always seal the pieces all over before adding a mosaic.

Cement foam boards are lightweight, strong, and suitable for exterior works. They’re easily cut with a box cutter or a power tool. When using this type of board, the edges need to be protected and hanging hardware needs to be installed before adding the mosaic. Check that the brand you choose is suitable for exteriors.

The back of porcelain floor tiles are useful as smaller substrates. 

Other suitable substrates, depending on the environment, include: cement sheets/backer boards (but these can be heavy, especially if wet); concrete; glass; slate; terracotta (which is porous and should always be sealed; salts can leach outwards from the soil in pots, impacting the integrity of your adhesives); or polystyrene (that’s been prepared).

Choose sufficient thickness to eliminate any flex. I prefer the look of thicker substrates, but keep in mind that this will also increase the weight.

With some substrates, you may need to consider sealing or waterproofing depending on what adhesive you’re using and the environment that it’s going to be in.

Avoid plastic as it can soften in the heat and tesserae could pop off. Over time, plastic becomes brittle and cracks. There can also be issues with finding a suitable adhesive to bond to plastic.