The surface that you choose to lay your tesserae on will be one factor in determining if your mosaic lasts more than a day. There are tons of possible mosaics bases out there but beware of inappropriate ones. Whether you choose wood, concrete, metal, or even plastic, you must know that it will hold your tiles. The best rule is that if it flexes then do not use it!
Wood is one of the most common mosaic surfaces because it is so readily available. Go to several yard sales and you will find tons of old furniture crying out to be used for mosaics. You can also buy unfinished wood shapes at various stores.
You do need to prepare your surface before tiling it. Either use a mixture of 1/5 Weldbond and 4/5 water or use a latex primer. Cover all sides and edges of the unfinished wood to protect it from warping. If your wood already has a finish such as polyurethane then you will just need to scuff it up a little so the adhesive will hold better.
There are several types of glue that can be used on a wooden surface. Some of the most common are Weldbond, Liquid Nails, and thinset mortar, just to name a few. You can also use any type of tesserae on this surface. Grout and seal as you normally would.
A concrete stepping stone, bench, or table are some of the most common forms of concrete bases used in mosaics. There is no need to seal a concrete form even though it is porous. Just be sure to use thin set mortar for your adhesive. It is the best glue for your cement surface. If you are going to use Weldbond then you will need to seal the stone with the mixture stated above.
Concrete pieces are great for setting outdoors as long as you use thinset. You can place these in your garden in any environment. The only drawback is that these are very heavy. However this could be a plus for a mosaic that you want to stay in place during inclement weather.
Another form of concrete is a backerboard or concrete board such as Hardibacker or Permabase. These are ideal for large wall murals. This board can also be used for outdoor projects. Just remember to affix your hanging apparatus before setting your tiles since it is too thin to nail or screw into.
A planter made of terra cotta makes a nice finished mosaic for your patio. Remember that terra cotta is very absorbent. You have to completely seal it inside and out. Thinset or Liquid Nails is the best adhesive for this surface. These can withstand any added moisture that your soil may introduce in the planter.
A helpful tip for tiling these would be to lay it in your lap or turn it upside down during the mosaic process. There really is no need to tile the bottom though.
Glass is very common because it is so readily available. Find this in the form of vases and pots. It is a little more difficult to add tesserae to a glass vase. The tiles tend to slip easily. You will need to use silicone or Liquid Nails as your adhesive.
To make a pretty votive cup, be sure to use a clear adhesive and cathedral glass. Cover the entire back of each tile so that you can not see the glue through the tiles.
There are many other surfaces available to use. Ceramic tiles, acrylic, metal, and fiberglass are great mosaic bases. You will just need to use the correct adhesive and surface prep for each one.
If you lay your mosaic onto the wrong surface then it will not last long. If the surface flexes then your tiles will pop off. If you tile something that will deceive such as wicker then you will ever have trouble. Just make sure that you are secure in your surface choice.
Creating a mosaic that will last forever is easy. Do you homework and check out all of the possibilities of success and failure.