Ceramic Tile Institute on Glass Mosaic Installation




12061 Jefferson Blvd., Culver City, CA 90230-6219

CTIOA Field Report 81-1-4 (R-89)


    1. The purpose of this report is to offer some insight into the challenges and peculiarities of installing.glass mosaics, especially murals.
    2. This report will outline procedures for installing glass mosaic murals.
    3. The types of murals that will be discussed are those made of both Smalti, Byzantine glass and Venetian glass.
    4. Murals made of glass mosaic, especially the Smalti type glass, are very costly and can easily be ruined unless they are properly handled and installed. For this reason, many specifications require the mural be installed by: "Professional Journeyman tile setters experienced in setting tile of this type under similar conditions". This field report, therefore, is as much a not-to-do warning for those inexperienced in mural installations as it is a review of procedure for those who are experienced.
    1. Both types of glass are installed basically the same way.
    2. As stated in a prior issued CTI Field Report, No. 66-2-9, "Backbuttering Glass Mosaics", a successful glass mosaic. installation cannot be obtained by combing thinset material over a hardened surface, such as a prefloat mortar bed, and placing the glass mosaic sheets directly on the thinset mortar. This procedure most surely will result in a failure. The major factors Contributing to the failure are:
      1. Insufficient contact of the thinset bonding mortar to the glass., resulting in little or no bond.
      2. An inability to beat the Byzantine glass in on a hard wall because all tesserae are not of equal thickness.
      3. Both kinds of glass must be back grouted, they cannot be face grouted.
      4. Not locking each individual tessera of glass into place by back-buttering the bondable side of the sheets of glass.
    1. Glass mosaics are best set in the mortar method of tile installation.
    2. The mortar method referred to is in the ANSI A108.1 specification.
    3. The materials are as follows:
      1. Portland Cement:Conforming to ASTM CISO, Type 1.
      2. Hydrated Lime:For masonry purposes and to conform to ASTM C-207 types, plus the added requirement limiting the unhydrated oxides to 8% maximum.
      3. Sand: ASTM designation C-144 washed plaster sand and specification for aggregate for masonry mortar ASTM C-144.
      4. Water:Free from impurity that is injurious to the construction and shall be fit for human consumption.
      5. Grout and Buttering Mix:Shall be a grout mix with up to one part sand to two parts portland cement. Sand to comply with CTI 70-6 Standard for Graded Extra Fine Sand Aggregate (See CTI Field Report 73-1-3 for further information on fine sand aggregate.) White sand complying with CTI 70-6 is available from Crystal Silica Sand Company in Oceanside, California. It is called Tile 70 Sand. The only way to get grey graded sand is to screen washed plaster sand using a 16 mesh screen (window screen).
      6. Mortar Setting Bed:Shall be mixed in the proportions of one part portland cement, one part hydrated lime, six parts washed plaster sand.
    1. Lofting is a procedure whereby the mural is laid out on a clean, dry surface, paper side down.
    2. The sheets are placed so that they fit tightly together.
    3. Measurements are then taken to make certain that the mural fits properly into the space it is designed for.
    4. Special care should be taken when lofting, to protect the sheets from moisture such as a damp concrete slab. Also, it must be protected from the direct heat of the sun. Moisture will begin to release the tile from its paper face. Excessive heat will cause the paper to curl and the glue to become brittle. Either condition could cause the tile to come loose from the paper face.
    1. Mortar bed should not exceed 1/2" in thickness. A plumb scratch can be applied to obtain desired thickness, or compensate for irregularities in the wall or floor.
    2. Setting the mosaic in fresh mortar provides the most favorable installation.
    3. An area not to exceed a days installation should be floated. An experienced journeyman can expect to set fifty square feet per day. This estimate is of course dependent upon job and weather conditions.
    4. The mortar bed must be firm before setting sheets. A wet or heavy bed could precipitate slipping, air pockets, and difficulty in working the sheets together. Mosaic sheets are one-of-a-kind units. If it falls apart or is ruined by putting it on too wet a bed, an expensive work of art may very well be irreparably damaged.
    5. Mosaicist draw a center line and also draw squiggle lines all over the back of the cartoon paper prior to cutting it into sections and mounting the Tessarae. These lines are used to plumb and level and keep details such as hand and faces straight and true.
    6. Constant care must be taken to make sure the mural is level and plumb. If it begins to drift out of level, it is difficult or impossible to correct the mistake.
    7. The backbuttering of the mosaic is of great importance for several reasons: One, it locks the individual pieces into place so that they do not move independently after the sheet is set. Two, it provides a 100% bond between tile and mortar bed. Three, it removes any foreign substance such as dirt or dust from the back of the tile that could act as a bond breaker. Four, the backbuttering process is also back-grouting the tile. Beveled Venetian glass cannot effectively be face-grouted.
    8. Extreme care should be taken not to butter the glass too far in advance of setting the sheet. Depending.on the weather, consistency of the backbutter and the experience of the setter, that sheet has between 30 and 60 seconds before the water in the backbutter effects the glue on the paper holding the glass together. If the water is allowed to dissolve the glue, the sheet could tear, fall apart, or become difficult or impossible to handle.
    9. The placing of the sheets is critical. Care should be taken to fit the sheets together as tightly and smoothly as possible. A few extra minutes fitting the sheets initially, will save hours of tedious work after the paper has been pulled from the face of the glass. This careful fitting of the sheets is to prevent sheet marks from showing where two sheets come together.
    10. Sheets should be tapped into place and joint lines smoothed before pulling the paper.
    11. Wetting the sheets is a variable. In hotter weather, fewer sheets would be set before wetting. The opposite, of course, is true in colder weather. Generally, between 20 and 30 square feet could be set before wetting the sheets and pulling the paper.
    1. The glass must be rubbed into place, using a block of wood or comparable implement. The face of the glass must be rubbed flat and smooth.
    2. When a mural is so large that it takes more than one day to install, the mortar extending past the edges of the sheets in place must be cut clean back to the scratch coat. This procedure is done so that the next days work can be fit nicely into place.
    1. This method has been related to us by workmen who have used it successfully in the field.
    2. The materials are the same as those specified in C.3 a-d. The mixing ratio for the backbutter is different. The backbutter mix is 3 parts sand 1 part portland cement and I part hydrated lime.
    3. This procedure is for setting Smalti Byzantine over a pre- floated setting bed.
    1. Mural is lofted and its actual dimensions are taken.
    2. The area is prefloated using the mortar mix detailed in C.3 f. The prefloating is done to accommodate the thickness of the glass plus 1/2".
    3. The mortar bed is then cover-cured until dimensionally stable.It is recommended that it be cured for seven days.
    4. A box screed is then made so that each sheet will have exactly 1/211 of backbutter on it in addition to the glass.
    5. A skim coat of thinset is then applied on a small section of the wall. Care should be taken that the thinset does not skim over before the sheets are applied to the mortar bed.
    6. Make certain that there is a good bond between the prefloated mortar bed and the backbutter. The shear strength of one coat of mortar applied to another coat of mortar without an effective bonding ingredient, such as a cement slurry or thinset, is very low.
    7. All other procedures are the same as those outlined in the previous sections.
    1. Refer to backbutter mix in section C.3 e. This same material is used to grout the glass.
    2. All cement and glue scum must be cleaned from the face of the tile prior to grouting.
    1. Glass mosaics are fragile works of art. Each step of the procedure must be taken with care by those experienced in doing this work.
    2. Lofting for dimension, backbuttering for maximum bond and careful placement and working in of the sheets are the most important aspects of a good installation.
    3. The installation of glass mosaic murals is perhaps the most difficult procedure in tile setting. It is for that reason that we recommended only professional journeymen with experience in dealing with this material under many circumstances be called upon for an installation.of this kind. There can be no short cuts when installing glass mosaics. The glass dictates its own speed, therefore, control of all of the materials is absolutely critical. Glass is most unforgiving. One simple mistake could ruin an expensive work of art and be very costly to the installer.




12061 Jefferson Blvd., Culver City, CA 90230-6219

CTIOA REPORT 2002-4-25

SUBJECT: Direct Bond Thin Set Method Glass Mosaic Tile Over Exterior and
Interior Portland Cement Mortar Beds (cured 7 days) and Cementitious Board Units (CBU)   

This Field Report has been written for the installation of paper faced glass mosaic tile, using a direct bond thin-set method with no requirement for back buttering the glass tile sheets prior to installation. This method can be employed on exterior and interior cementitious substrates, using manufacturer recommended setting systems, combined with specific trowel size and technique.

In 1985, Field Report # CTI 66-2-9 (R85) described back buttering as an essential part of all paper faced glass mosaic tile installations. Since this time millions of square feet of paper faced glass mosaic tile have been put into service using the direct bond thin-set method. With the increasing volume of glass tile sales, combined with the versatility and ease of the direct bond method, the tile industry will benefit by supporting a direct bond method.

Laboratory tests have determined using the direct bond thin-set method, on cement mortar beds, cured 7 days, can achieve higher bond strengths than the wet set or modified grout back butter methods. Additionally, this method has proven to be successful when bonding to cementitious backer units.

Consult glass tile manufacturers, as well as, tile setting adhesive manufacturers for their recommendations regarding specific installation material compatibility.

For installations such as murals having tiles of varying thickness, glass mosaics thinner than 3/16”, and domes (concave and convex), the back buttering technique and box screed methods are considered the preferable techniques.

Glass tile has a high degree of expansion and contraction requiring the installation of expansion joints. Size, location, and type, must be designated by an architect and shown on drawing details. Typical locations include but are not limited to exterior 12’ on center, between adjoining dissimilar materials, ceilings, and the perimeter. For additional recommendations regarding expansion joints, including applications for extreme temperature variations, refer to ANSI specification
A108 AN 3.7 thru

Framing (Wood or Metal)
Maximum variation in backing surfaces 1/8” in 8’ from the required plane. Metal framing should be installed according to ANSI A 108.11-1999

General Substrate Preparation Recommendations
All substrates to receive paper faced glass tile should be designed to have a deflection rating of not greater than 1/360 of the span. Make allowance for live load and impact, and all dead load including the weight of tile, grout, and setting bed.

*Caution: Never set glass tile directly over wood products.

(Best for exterior freeze thaw areas) New construction light broom finish, cured 28-day minimum. Install over clean, sound, dimensionally stable, existing masonry or concrete. Do not install over cracked or coated surfaces without proper preparation.

Wire Reinforced Mortar Beds
(Not recommended for exterior freeze thaw areas)
Follow ANSI A108-B (7day minimum cure time prior to installation).
Please specify 3.4 lb. Metal lath on approved steel stud framed applications.

Cementitious Backer Units (CBU) Interior
Acceptable CBU standards ANSI 118.9, ASTM 1325
Fiber cement under-layment ASTM C-1288
Follow manufacturers recommendations regarding fasteners.
*Important: Install CBU’s to metal framing following ANSI A 108.11-1999.

Submerged and Below Grade
Cure minimum 21 days after grouting, prior to submersion or heavy water use.

Material: Paper Faced Glass Mosaic Tile
During the past decade the tile industry has seen significant growth in paper faced glass tile products. Many of these products resemble the original glass mosaics manufactured in the 1950’s, featuring surfaces and textures with a variety of characteristics, including chips and other minor imperfections that make hand made glass tile desirable. Note: the term “without chips”, as noted in CTI 0-106-68, 3. may conflict with products being specified today. In addition to different shapes and sizes, there are tiles of different thickness, which suggest (1) of the same specification should be changed to, *Tessarae shall be cast in uniform thickness, and uniform size. Specification writer must supply information regarding the particular type and quality of material to be installed.

Installation Adhesives for Direct Bond, Paper Faced Glass Mosaic Tile.
Not all 118.4 latex modified thin-sets, whether spray dried polymer or two-part bonding mortar systems, are recommended for installing mosaic glass tile. However, all of the cement based setting materials recommended for glass tile installations by glass tile manufacturers, as well as, adhesive manufacturers are 118.4 rated. It is the responsibility of the writer of the specification and the installer to confirm the use of recommended setting adhesives as accurate for their intended use. Recommended setting temperatures for these types of setting materials is generally 50-90 degrees Fahrenheit. Keep installation surface out of direct sunlight, and wind, using screens, tarps, and umbrellas, to reduce the problem of skinning over (slight drying of the setting material surface) during installation.
*Caution: Do not mix adhesives over 150 RPM.

Paper faced mosaic glass tile should be set by journeymen tile setters experienced with setting paper faced mosaics under similar conditions. Determine proper trowel size by measuring the glass tile thickness and follow the graph below to determine the appropriate notch trowel size and style.

Glass Tile Thickness

Trowel Size


1/8” square notch


3/16” x 3/16” square notch


3/16” x 1/4” V-notch

3/8” & thicker

1/4” x 1/4” square notch


The preceding article was researched and written by Scott Fleming, CTC. We wish to thank Mr. Fleming for his excellent report.


Installing Paper Faced Mosaic Glasstile Using the Direct Bond Method.




Using the flat side of an appropriate sized V-notch trowel to initiate the bond coat, firmly apply the setting material into the substrate.


With additional setting material, using an appropriate sized V-notch trowel, comb the notches full to establish the proper depth of the setting bed.





Using the flat side of the trowel, flatten the notches to achieve a smooth, consistent setting bed.


Apply Tessera mosaics into the setting bed (paper side facing you) using light, even pressure to establish contact


Installing Paper Faced Mosaic Glasstile Using the Direct Bond Method.




To achieve a uniform, flat surface tap lightly, using a wooden beating block and a hammer.


Apply subsequent sheets lining up grout joints from one sheet to another.





Unify sheets by tapping lightly from one sheet to the next. *Tip: Do not use rubber floats or steel trowels for this step or step 5.


After 15-30 minutes (floors can be removed sooner) lightly wet paper several times over a 5-10 minute period. When the paper turns dark it is ready to remove.



Installing Paper Faced Mosaic Glasstile Using the Direct Bond Method.





Removing paper prior to final set allows adjustments to achieve a consistent overall field. Pay particular attention to the joints between sheets to eliminate sheet pattern.


Cure a minimum of 24 hours prior to cleaning with a nylon scrub brush and warm water to remove residual paper and glue, followed by wiping clean with a damp sponge.





Using non-sanded grout for joints 0-1/8”, or standard grade sanded grout for back beveled glass and joints from 1/8”-1/2” apply grout with a rubber grout float forcing grout into joints until full.


Grout joints shall be full and uniformly finished. Due to the impervious quality of glass, grout will take longer to set than with more porous tile. For initial cleaning use dry cheese cloth to wick moisture from grout, as well as avoiding washing the grout out.


Installing Paper Faced Mosaic Glasstile Using the Direct Bond Method.





After cheese clothing allow grout to set up followed by smoothing the grout joints with a lightly damp sponge.


For final polish of excess grout haze, after grout has cured, use clean, dry, cheese cloth.


Cleaning Recommendations: (based on cure times)

  • 0-24 hours, no additional cleaning.
  • 24 hours to 10 days, warm water and neutral ph cleaner specifically designed to clean tile.
  • Cement based haze, grout cured a minimum of 10 days and longer, sulfamic acid cleaning crystals mixed with warm water using a nylon scrub brush following manufacturers directions rinsing with clean water.
  • Latex based haze, grout cured a minimum of 10 days and longer, as well as, areas having other materials that may be adversely affected by acid based cleaners, such as, polished stone, soft glazed ceramics and metal plumbing fixtures use Stone and Tile Stripper.
copyright George Fishman 2017