When making a mosaic, it is important that all three components; the mosaic materials, the substrate (surface the mosaic is attached to) and the adhesive are all compatible with each other and appropriate for the siting of the mosaic. For instance, some mosaic materials such as china and substrates such as wood are more appropriate for indoor use. Adhesives are also rated for indoor or outdoor use. There is no one answer to the “which glue do I use?” question. Adhesives are rated for different uses such as indoors, outdoors, horizontal, vertical and submersible. It is important to choose the right one for your application
PVA Glues (white glues)
PVA glues such as Weldbond, Selley’s Aquadhere etc are easy to use, dry clear and are non-toxic. However, as they are water soluble, they are not suitable for mosaics which will go outside or live in very wet areas. They are good for indoor pieces and also for when using the direct method with fibreglass mesh.
These are usually suitable for outdoor use. They come in many brands and tile shops often have a good variety. They are available in powder form, and usually in either grey or white. Some are mixed with water, others are two-part and are mixed with a polymer-fortified latex additive to give them flex and strength. The pre-mixed ones are usually not as good as the ones you mix yourself. Cement-based adhesives can be coloured with oxide powders, but it can be difficult to maintain consistency over many batches. Some people use acrylic paint but this is not recommended as it can weaken the mix and may fade over time.
It is important to choose the right one for your application, some are rated for outside use, some for both horizontal and vertical applications and some for submersible ones. There are various reputable manufacturers available in Australia including Ardex, Mapei and Laticrete. Laticrete Gold, Mapei Kerabond with Isolastic additive, Ardex Optima and Ardex X77 are among those which many people use for mosaics. As different adhesives are rated for different purposes, we would strongly recommend that once you have identified what you would like to use, you check the technical data sheet on the manufacturer's website to see if it is suitable for your purpose, or ring the manufacturer's technical support line.
Note: Cement based adhesive that can be laid in a thin bed are called “Thinset” in the USA and this term is often used in mosaic books.
As well as adhering the mosaic to the substrate, cement based adhesive can be used to cover the sides of marmox board, and also for putting a skim coat over the face of the board if required.
Cement-based adhesives are rated for specific uses. Eg; one might be rated for use with ceramic tiles for internal vertical use, but not for outside horizontal use. So you could use it inside on a wall but not outside on a floor. Ratings will also tell you whether the adhesive can be used in a wet area, such as a shower, or for submersible use such as a swimming pool.
Normally, grout has no adhesive qualities and is just used to fill the gaps. However, several manufacturers including Laticrete and Davco produce additives which turn cement-based grout into a cement-based adhesive giving you an instant coloured adhesive if you use coloured grouts.
All cement-based grouts and adhesives need to slake before using. This is a rest period which can be from 2-10 minutes depending on the brand which allows the chemicals in it to interact with one another before use. After the "slake" period remix slightly then apply.
There are other adhesives such as two–part epoxies and silicone based glues. These can be tricky to work with and may also give off fumes. Not suitable for beginners.