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Preparing the space to install mosaics
Set the mosaic on a substrate that is rated to hold natural stone. This will include cement boards, traditional mud beds,
etc. Consult a professional builder on this matter before starting the project.
There must be at least one and one-eighth inch of substrate to support the mosaic. Make sure the final height of the mosaic
including substrate and mortar will be the desired height at the completion of the job (normally the same height as the adjacent
floor). If you are tiling under installed cabinets, ensure there is adequate room to install appliances.
Consider where the finished surface of the mosaic should be. Will the mosaic panel be even with custom moldings or a mosaic rug,
even with an existing hardwood floor? Whatever the case, this should be planned before nailing in any substrate. To achieve the finished
surface desired, it may require building walls out or even recessing the surface on which the mosaic will be installed. There will be an
inch and one-eighth substrate, perhaps a sixteenth-inch of setting material, and at least three-eighths for the stone mosaic.
Ensure the framing is structurally sound and up to code. If a type of cement board is used and the mosaic is larger than one piece
of the board, the board may have to be cut in several pieces to fit the space. Tack the substrate up with one and one-quarter inch
corrosion resistant roofing nails. Fill all the joints with the same mortar used to set the tile. Embed the two-inch wide, fiberglass
cementitious backer unit tape in the mortar, and level.
Be sure the entire surface is level. Each piece of board should be flush with the next, all cement floated properly, etc.
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Dry-laying or "fitting" the mosaic
First, dry-lay the mosaic in the area being prepared for it. Templates can be wrong; cabinets moved, walls shifted, etc. This
will ensure the mosaic is going to fit before it is laid in the mortar. Finding out if the size needs to be adjusted before
setting with the mortar affords the opportunity to properly and evenly adjust the mosaic to fit the space or adjust the space
to better fit the mosaic. New Ravenna Mosaics provides "maps" keyed to the marked sections of tile.
Client approval must be confirmed once the space and the mosaic are adjusted to work with each other. If the mosaic is unacceptable
to the client under the tape, it will not likely be approved after the installation.
Once a manageable size has been determined, cut and connect the pieces. Make notes and marks so that each section is identified
within the space. A good method is to trace around each piece or section on the floor or wall then number each piece on the space
as well as on the mosaic. This gives a reference for where each section should go. This is an important part of dry fitting. Mosaics
being made out of possibly several hundred tesserae per square foot are very flexible, and can stretch and shrink easily. The pieces
must be set back exactly where they were when it was dry fit to avoid having as much as a several-inch deviation of the mosaic to
its intended space.
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Setting the mosaic
Remove the mosaic from its dry lay position and flip each section face down. This is TAPE SIDE DOWN. There may be clear contact
paper on the back of the mosaic. It is very important to carefully pull this off the back of every piece. Set the sections aside
remembering which piece is where and how they all fit together.
Thoroughly clean the surface on which the mosaic will be installed, sweep all debris off the surface, vacuum and wipe it down with
a damp sponge. After the surface is completely clean, inspect the surface for any dips or bumps. Make all repairs necessary to make
the surface level and smooth.
Prepare the mortar. Latex modified thinset is suggested. Be sure to use white thinset. Ensure that whatever type of mortar used
is rated to hold the types of stone that are included in the mosaic. Follow the package instructions to make it ready to be troweled
onto the surface.
Using no larger than a 1/4" x 1/4" V-notched trowel (a 3/16" - "V" notch should be the minimum size used so that differences in
tessarae height can be accommodated), trowel out enough mortar to cover the area to set the first few sections. NOTE: Slight
differences of up to 1/16" in height are common as the marble is sourced from quarries all over the world and there is no
universal standard. The lines traced around the sections will serve as a guide. When troweling the mortar out, be sure that it
is spread out evenly. Thick spots will result in mortar bleed up through the grout seams that will then have to be picked out
below the surface before grouting. Thin spots will result in tesserae not sticking at all.
Carefully place the first two to four sections (tape-side up) into place. Using a level surface (such as a block of wood) and a
rubber mallet, tap the sections into the setting material, avoiding the outside edges that will abut the next sections. Then press
the next sections into the setting material, making sure that they are as close to the first sections as possible. Doing this is
extremely important so that the seams don't show. Use the grout space between the tesserae within the section of mosaic as your
guide for space between the taped sections. Tap again with your flat level surface (such as a block of wood), this time pressing
down on the seams between sections (this avoids "cupping"), but again avoiding the outside abutting edges (except for the edges
that are against the outside wall or threshold).
Repeat step (5) on the next few sections. After several sections are set to level, closely examine the seams in between each of
the sections. If the seams are undetectable after careful examination, the job has been done well. Seams are created by gaps in
section placement, uneven (height) pieces along the edge of the seam, and tesserae not lining up within a section, etc. The seams
will need to be re-worked between the sections as necessary to make disappear. Techniques to rework seams include but are not
limited to: cutting through the tape with an X-acto™ or utility knife to adjust sections of tesserae around the seam; extracting
and resetting pieces on the seam to blend them in. Repeat steps (4) and (5) until the mosaics are entirely set and seamless and the
installer and the customer are completely satisfied with the installation. Follow the instructions on your mortar package; do not let
anyone go near it for 24 to 36 hours, depending on the type of mortar chosen.
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Pulling the tape and repairing loose tesserae
Once the mortar has dried and the tesserae have firmly adhered, the tape must be carefully removed. Begin at any convenient
spot and start peeling the tape backward on itself. It is sometimes helpful to use an X-acto or utility knife to get a corner
of a piece started and then pull the rest by hand. If many pieces come up with the tape, you have either not allowed enough time
for drying or not applied enough thinset underneath.
As the tape is pulled off the face of the mosaic, loose tesserae will likely come up. This is normal. When you encounter the
spots where there are only a couple of loose pieces, just set them next to the hole they came from and continue to pull the tape.
For larger sections that are loose (this is not normal), try and cut through the tape around the loose section and extract it all
at once to avoid having to piece it together, one tesserae at a time.
After pulling all the tape and setting all the loose tesserae next to the spots they need to fill, thinset the loose tesserae back
into place. First, ensure that the pieces will fit back in the holes and set flush with the chips around it. The cavity may need
to be cleaned out until the chips fit properly. Once a perfect fit is achieved, mix enough thinset to fix the spot. Back-butter
the pieces with thinset, then set them into place. Repeat this until all the holes are filled. If a rapid setting thinset is used,
the mosaic will be ready to grout in 1 to 2 hours. If regular thinset is used, then it will be ready in 24 hours.
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Grouting the mosaic
Before grouting, some tesserae will have a slight film on their surfaces and will appear cloudy. This film will disappear
after grouting and final cleaning. If your mosaic is a tumbled or honed finish, apply a grout release only to the tops of
the tesserae.
Grout as usual.
Clean the joints with a damp sponge, taking care not to remove too much grout. A powder residue will appear. Buff
this with a soft dry cloth.
If grout dust still remains on the tiles, use water and rinse repeatedly until the dust is gone.
Occasionally, tape residue will be left on the surface of the tiles. If this is the case, use NAPA adhesive remover
(available at NAPA auto parts stores) or a similar product and it will dissolve easily.
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About back-buttering
Sometimes, tesserae will be of slightly different thicknesses; this happens if glass and marble are mixed, for example.
To even what will be the top surface of the mosaic, a technique called "back-buttering" is used.
Tesserae mosaics are face-mounted on tape. Place tesserae tape side down on a flat surface. Spread powder (dry mortar)
into grout joints with a foam paintbrush. Fill the grout joints, leaving no powder on the tesserae. Mix dry mortar and
acrylic latex additive to the proper texture (thick enough to stand up when a trowel/spreader is passed through). Use
the flat side of a plastic trowel to carefully back-butter tesserae. Spread a thin layer of mortar to cover the tesserae.
Let dry for 24 hours.
About mounting
Prepare the mounting surface as needed. It should be stable and free of cracks, waxy or oily films, curing compounds, dust,
or any other substance that could act as a potential bond breaker. Mix dry mortar and acrylic latex additive to the proper
texture (thick enough to stand up when a trowel/spreader is passed through). Apply a bonding mortar coat to the substrate
with the flat side of a trowel/spreader, pressing hard to attain good contact. Comb the mortar with the large tooth edge
of the trowel/spreader to achieve a uniform coat. Recomb the mortar bed with the flat side of the trowel to render a flat,
ridge-free, and uniform application. Spread only as much bonding mortar that will allow for installation of the tesserae
and tape while bonding mortar is still in a fresh condition and prior to its initial set.
Mount each section with the tape side up. Apply bonding mortar to the substrate as explained above. Begin mounting slabs
in small sections (approximately 4-10 square feet). Firmly press each slab into the setting material, while pressing down on
the one next to it to avoid "cupping" of edges, checking to make sure there is sufficient coverage with the setting material.
Then beat it in lightly with the rubber grout float or wooden block. Let stand for 24 hours, and then remove tape. Brush powder
out of grout joints with a stiff bristle brush. Clean the surface with adhesive cleaner to remove any tape residue. Then seal the
tesserae with grout release (to prevent grout from penetrating). Allow time to dry.
About grout
Certain marbles may be discolored by grout. These marbles should be sealed or treated with grout release prior to grouting.
This includes: All tumbled marble, all honed marble, and all limestone, such as Fontenay Claire, St. Richard, St. Vincent, and
Ivory Cream. Any light colors that might be porous, such as Calcutta Tia, Rosa Portagallo, Thassos, and Celeste.
See "Grouting the mosaic" above for more information.
Suggested materials
Mapei - KER-102 - Kerabond
Premium Floor and Wall Dry Set Mortar
Mapei - KER-310 - Keralastic
Flexible Acrylic Latex Additive
Mapei - KER - Granirapid
Premium Rapid-Setting Flexible Mortar System
Mapei - KER-318 - Granirapid Liquid Flexible Admixture
Mapei - KER-800 - Polymer - Modified Wall Tile Grout
Regent Stone Products - Silicone Impregnator (sealer)
NAPA - Universal Adhesive Cleaner
Plastic or metal 1/8" - 3/16" V-notched trowel/spreader
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