As I live in central Scotland, I’ll always recommend materials for that environment, taking into account rain and frost in particular. If you’re lucky enough to live in a sunny climate all year round, your list of possible materials will be bigger than mine!
Substrates
These include:
- Cement foam board (comes in a range of thicknesses and you can cut it into any shape)
- Slate
- Polystyrene
- Cement bases / walls etc
- Metal bases
- Porcelain tiles
Top tips:
- Don’t use wood as your substrate as water makes it expand and contract, and eventually all your mosaics will fall off. I was first taught to use marine plywood but I’ve discovered that even it might get water logged and your mosaics may be impacted. So you’re better to stick with one of the options above.
- You’ll need to prepare cement foam board and polystyrene before you start to attach your mosaics, and I’ll do another post soon covering how to do that.
Tessarae
“Tessera” (plural is tesserae) is the Latin word that describes any small piece of stone, glass, ceramic, or other hard material cut in a cubical or some other regular shape in mosaic work. You have lots of choices for exterior mosaics, but they need to be durable for all weather conditions. Here are some dos and don’ts…
| Do use: | Don’t use: |
| Glass tiles, stained glass and glass beads/gems | Plastic beads |
| Exterior grade ceramic tiles / shapes | Low-fired pottery / ceramics (non porous) |
| Highly-fired pottery / ceramics / bone china | Terracotta pots |
| Galvanised metals | |
| Mirror (prime the back with exterior PVA) | |
| Smalti | |
| Millefiori |
Top tips:
- When you’re buying any tiles / beads / ceramics for mosaics, read the small print and check that they’re suitable for exterior use – and particularly that they’re frost proof.
- The reflective coating on the back of mirror sometimes gets damaged when outside, but priming the back of it with exterior PVA before adhering it to the substrate will help avoid this. I still choose to use mirror because of its wonderful effect in exterior mosaics.
- You can use glass glitter tiles (or any clear glass tiles that have a surface underneath the clear glass), but you should note that they fade in sunshine. I still love to use them in mine.
- You can tell if ceramics are suitable high/low fired by putting a few drops of water on the back. If, after a few minutes, the water is still there then they’ll be ok for exteriors. If the water has soaked into the ceramic then you shouldn’t use it outside.
Glues
The best thing to use is exterior grade cement tile adhesive (or “thinset” if you live in the US). You can get this easily in DIY shops or builders’ merchants.
Top tip:
- If you get white adhesive, you can use it for transparent glass (which helps the light shine through the glass). And if you buy black cement dye, you can add that to the adhesive when you need a darker colour.
When I first learned mosaic, I was taught to use exterior quality PVA. While I haven’t had major issues with it, and my mosaics have stood the test of time, other mosaic experts I’ve learned from since say it’s best to avoid it for exterior mosaics because it can turn back to it’s original state if it comes in contact with water somehow. So I now use, and recommend, exterior tile adhesive for glueing pieces. However, I still do use exterior PVA for some things, like priming the back of mirror before using it in exterior mosaics.
Some people also use silicone and I’ve experimented with it, but I mostly use tile adhesive. Silicone is especially good for glass-on-glass mosaics.
Grout
You need to use exterior grade grout.
Top tip:
- You an experiment with colours, but you should note that your grout will get dirty being outside. Black is great for making tiles “pop”. A mid-grey is a safe bet and has a more unifying effect than black. Your grout colour will depend on the type of effect you’re going for.
Rhona Duffy 