Mosaic Adhesives

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How to - Mosaic Adhesive Tips

Choosing the right adhesive for your mosaic

With are so many adhesives, so many backing materials, and tesserae to choose from, choosing an adhesive can be very confusing. This section will help guide you in your selection of what to sticks to what. Keep in mind this is only a guide. What works best for one person may not be favored by the next. My best advice is to read the product label, experiment a bit and find what works for you always keeping in mind the longevity of your final work. Below you will find a list of the most often used adhesives in mosaics. There are certainly other choices, but with this list you will have a solid base for making decisions as you go forward creating.

Thinset

Thinset is a cement-based adhesive you mix with water OR a liquid polymer additive for added flexibility and durability. It is the same product used in construction by tile installers. There are many different manufactures of thinset and each manufacture may even have several types. It is extremely important that you familiarize yourself with the product that you are purchasing and select the one that is appropriate for your project application. Read the bag! There is a wealth of information included and products vary. You can also visit the manufactures website to learn more.

Please note that the setting or “drying” process of thinset is not a drying process at all. It is a curing process that happens through a chemical reaction between the thinset powder and water. Too much or too little water can affect the quality of the cure of the product. Thinset is better and stronger if it is given an extended period to cure. Hot days and sunlight are not your friend, nor are porous backers that wick the moisture out to quickly. It is a good practice to cover you work at the end of each session with a slightly damp towel and plastic, to keep the curing process going. (This applies to grout as well). Not all thinsets listed below are chemical cures. Each is noted.

Thinset comes in three main types.

Non-Modified – This means that no polymers are added and most manufacture will recommend that that you use an admix in place of all or part of the water for higher bonding strength and flexibility. (Chemical cure)

Polymer Fortified/Modified – These are the most widely used among both artists and the tile industry. These products already have the polymer in them and are meant to mix with water. (Chemical cure)

Epoxy Based Thinset – Epoxy Thinset is quite different from cementitious thinset. Made from epoxy resins and a filler powder. Epoxy thinsets offer greater bond strength and chemical resistance than polymer modified cementitious thinset. This performance comes at a price, epoxy thinset is much more expensive than regular thinset. Typically, they are only used to bond to difficult substrates or where extraordinary chemical resistance is needed and there are drawbacks. They are mixed from two parts and must be mixed in precisely correct proportions to set properly, they are more difficult to work with and have a shorter pot life. Although this is a chemical cure the rules above do not apply, as this is a reaction between the two parts of epoxy.

Premixed thinset – This a ready to use no mix, no-mess alternative to traditional thinset mortars and a great choice for interior artwork where no water is present. It is easy to use and ready when you are, if the container is kept sealed it has an extremely long shelf life. Unlike traditional thinset Premixed Thinset is air-cured and using it to adhere tile Larger than 8” x 8” will significantly lengthen its dry time as will as will installations where the tile is face-mounted with plastic tape.

With all the above types of thinset there also a subset of rapid setting vs. standard set time. Rapid set thinset is rarely a good choice in the art world unless you are installing on a vertical wall and are VERY well prepared to apply your pre-mounted mosaics quickly. The work time is substantially shorter.

Another consideration is color. Thinset comes in two colors, white and gray. Both perform identically so the choice is made based on the material you are using and whether or not you will be grouting or tinting it.

ALWAYS where a dust mask or respirator while exposed to thinset is in it's powdered state. Once mixed with water it is safe to remove. Here’s why, once inhaled the thinset comes in contact with your respiratory system which is full of moisture. The moisture (water) is the last ingredient that the thinset needs to start getting hard and you don't want lungs full of hard cement!
DO NOT empty or clean your tools into the sink or drain. Remember thinset will harden underwater and in your pipes! The only person who will be happy about that is the plumber! I use a slop bucket to clean up. Once the water is to "icky" I pour off the top liquid in a corner of my garden and allow the rest to dry out. Then you can just bang out the contents of the bucket in the garbage.

Only mix as much thinset as you can use in one two hour work session. Once mixed it will not keep until the next day.
DO NOT add water once the mixture has set more than 5 minutes. If your thinset is getting to hard to work with mix a new batch, adding more water will make it weak. Remember the curing process, adding water interrupts it. A tip: Working it by moving it around by kneading or stirring will help to soften it a bit and give you more work time.
Now that I have you wondering why you would ever use it, I have to tell you thinset is always my adhesive of choice for exterior applications and most interior installations unless I am working on a backing that requires a clear or flexible adhesive. It works, it's permanent, and if they can tile your kitchen floor and the sides of buildings with it, it will certainly work for anything you will make!
My best thinset tip - I mix my thinset in an old sour cream container. That way I don't have to clean it up I can just toss it. Once mixed I place the thinset in a sandwich size Ziploc bag. You can then nip off the corner and apply it to your surface like using a cake decorating bag, just spread with a palette knife.
Thinset is your best choice if your base is concrete, terra cotta, cement backer boards, Wedi board and most permanent installations.
It is the only adhesive you can use to really 'float' tiles. This means you can build up the adhesive under a thinner piece of tile so that it is flush with the thick tile next to it.
It is best to seal cement, brick, terra cotta and other porous surfaces before applying thinset. These materials will wick (draw) the moisture out of the thinset to quickly and weaken it. I recommend Kilz, but it is always best to consult specifications for the product you are using.
Unused thinset in its powdered form must be stored in an airtight sealed container. If left in the open bag it will absorb moisture from the air and begin to cure. You will notice somewhat hard clumps of varying sizes. The clumps will be easily broken down with your fingers however once this happens it is no longer useable and will never be as strong as it needs to be. Check it for clumps even if you have just purchased a new bag. In the bag it still has a shelf life and/or it may not have been properly handled prior to your purchase.

PVA Glues

There are many brands of PVA glues. You know them as Elmer's, Tacky Glue, etc. The most widely used and preferred by mosaic artists is WeldBond. As I go forward in this section Weldbond is the product that I will be referring to and the one I recommend.

Non-Toxic, safe for children and fume free.
Dries clear. Great when bonding transparent and semi-transparent materials such as glass to glass.
Bonds to most anything. Glass, marble, terra Cotta, wood, Formica, etc. It is always beneficial to score (rough up) smooth surfaces to give them a little tooth. This process creates a better bond when using most types of adhesives. You can scratch them up with a sharp object or sand with a coarse grade of sandpaper.
Cleans up with water while it is still in it's "white" stage. High temperatures and low humidity will speed the curing process. The cure is complete once it has turned clear. It will continue to strengthen over time.
Works well to adhere tesserae to mosaic mesh when using the mesh method. It can be applied directly from the bottle.
My preferred method of application is to work from a small snap lid container and paint it on with a small paintbrush. This allows the glue to tack up a bit while in an open container.
When using to adhere glass to glass the lack of exposure to air will lengthen the dry time and the larger the pieces that you are gluing the longer it will take to cure clear.
Read the manufactures instructions! For further info about Weldbond that are not answered here, see the manufacturers specifications available at Frank T. Ross