Glass-on-Glass Tree Mosaic finished with grout.

Glass-On-Glass Mosaic Grout Color

Artist Laura Adams emailed me for advice on selecting a grout color for her glass-on-glass mosaic, and it is a good case study for several reasons.

First, the sky of the mosaic is a whitish gray.

Second, Laura made sure to photograph the mosaic with two different lighting regimes: backlit from behind and regular lighting from the front.

Given that a glass-on-glass mosaic looks very different when backlit, it wouldn’t be possible to make an informed choice without taking both situations into account.

Grout Gap as Preview

Many times the appearance of the unfilled grout gap gives you an idea of what the mosaic will look like with grout of a certain color.

For example, in the photo below, we can see what the mosaic will look like with darker grout in the sky area because the shadows in the narrow grout gap look like dark grout:

Glass-on-Glass Tree Mosaic, Ungrouted
Glass-on-Glass Tree Mosaic, Ungrouted


When does grout gap NOT give you a good idea what a mosaic will look like when grouted? The answer is: when you use a wide grout gap.

In an un-grouted mosaic with wide grout gaps, a lot of the color you are seeing in the image is from the sides of the tiles. Once you grout that mosaic, all the color from the sides of the tiles is covered up with dull concrete, and the mosaic is less colorful.

That is why professional mosaicists use narrow grout gaps. Novices tend to use grout gaps that are wider than they realize, and then they are shocked and unhappy with their mosaic after grouting.

TIP: Mosaics with narrow grout gaps are easier to grout, and their images are less effected by grout and the choice of grout color.

Glass-on-Glass Tree Mosaic, Ungrouted, Backlit
Glass-on-Glass Tree Mosaic, Ungrouted, Backlit


In the above photograph, we can see some idea of what the mosaic would look like with white grout because the backlight coming through the gaps looks white grout. It is important to note that the translucent glass is also more vibrant due to the backlighting. Compare this photo to the previous one.

What this photo doesn’t show is what the mosaic would look like with white grout when not backlit.

To visualize what this mosaic will look like with white grout when not backlit, you have to take into account that the glass colors will be less vibrant because the light will be reflecting off the surface instead passing through from behind and making the glass glow.

I have written quite a bit about the problems of white/light grout colors and the advantages of black/dark grout colors.

I don’t think that Laura’s mosaic would look very good with white grout or a light grout because the sky is a light color, but that isn’t the only reason. The main reason for me is that whenever the mosaic isn’t well lighted or backlit, a white grout will be more intense than the colored glass in the mosaic.

Exceptions to Rules

I will be writing an article about some mosaic photos sent to me by artist Jill Gatwood. Jill’s photos are of mosaics grouted with white grout, and were submitted as counter examples to my recommendations against white grout.

Please email me counter examples to anything I recommend. That sort of dialog helps tease out the subtle details.

Materials and Methods

Laura made her mosaic on a repurposed window frame with grill-and-panes construction. Repurposed window frames of grill-and-panes make good backers for glass-on-glass mosaics because they satisfy several requirements:

  1. The glass backer must not be too thin.
  2. The glass backer should have a protective frame around it.
  3. The grill provides support throughout the mosaic, keeping the ratio of width to backer thickness low.

Remember, glass can shatter on impact. It doesn’t make sense to spend so much time creating a mosaic on glass that is too thin. Mosaic tile and grout are heavy.

Sometimes glass-on-glass mosaic can break from their own weight during handing if the backer isn’t thick enough or there isn’t a frame adding support at the edges.

For this reason, repurposed window frames of grill-and-panes construction are a good choice for backer for glass-on-glass. You also reuse something instead of sending it to the landfill.

You should put a coat of paint over the frame and grid to cover any old lead paint. (Many old wooden window date from before the 1978 ban on lead paint in the U.S.)

A coat of paint also reinforces old hard window putty and helps it stay attached, which is an issue with the older frames, especially the interesting ones with all the weathering and decades of putty and paint repairs.

Adhesive for Glass-on-Glass

The adhesive we recommend for glass-on-glass mosaics is GE Silicone II Clear or a comparable brand of clear silicone adhesive.

We don’t recommend our conventional mosaic adhesive for glass-on-glass mosaics because Weldbond and other water-based PVAs cannot fully harden and turn translucent when sandwiched as a thin layer between sheets of glass.




10 responses to “Glass-On-Glass Mosaic Grout Color”

  1. Sandra Forrest Avatar

    Very good and informative article. I also make glass-on-glass mosaics. I love the window idea. The adhesive I use is the one you mentioned–the GE Silicone, but I prefer Dazzle Tac from Beacon. It has a quick grab quality so I don’t have to worry about the pieces slipping around. If I change my mind about a piece, I can use an Xacto knife to pry it up (at least for a few days until it hardens). And the narrow nozzle is easier to work with on small pieces.

  2. Maggie Donson Avatar
    Maggie Donson

    I love is project! Well done to Laura! So can you tell me was it done panel by panel… without removing the panels? I do stained glass and have buckets of small pieces of glass I would love to use up! This looks like a great way of doing that!!
    Thank you!
    Maggie Donson

    1. Laura Adams Avatar
      Laura Adams

      Thank you Maggie. I just drew the picture on paper and put it under the whole window (I did not remove the panels) and siliconed the glass I cut according to the picture I drew, then I grouted and sealed the mosaic. It was my first mosaic and first time cutting glass so it was a bit of a learning process, but I did enjoy doing it.

  3. Bill Dluzniewski Avatar
    Bill Dluzniewski

    What about using sheets of plexiglas instead of real glass ? It would avoid all of the problems using glass backer it seems. Would it work ?

  4. Bill Dluzniewski Avatar
    Bill Dluzniewski

    If you used plexiglas I would think you could also use weldbond as the adhesive, then on the backside of the plexiglas you could trace all of the “grout” lines with an acrylic paint pen thus achieving the groit effect on the glass side of the panel and you would still get the color from the side of the glass tile giving a brighter glow . What do you think ???

    1. Joe Moorman Avatar
      Joe Moorman

      I’ve had customers tell me that Weldbond adheres to plexiglass, but I’m not sure I believe that because plexiglass is an acrylic, and Weldbond doesn’t adhere to plastics, at least not very well.

  5. John Balch Avatar
    John Balch

    Beautiful work. I finished a glass on glass, and have it hanging in a window. I used white grout, in the evening the grout looks white. During the day when the sun hits it from the outside it looks dark. I’m glad I chose white.

  6. Tina M Maruco Avatar
    Tina M Maruco

    Do u really need to grout a gog if in a frame then hanging inside a window? Ty! Tina

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Tina,
      No you don’t, but the grout line in a G-on-G mosaic might be desired for aesthetic reasons. Like the lead-channel seam in a stained glass panel, the grout line serves as an opaque border around each glowing shard of glass.

  7. Ellis Eisner Avatar

    I made a glass-on-glass mosaic in 2019-2020 which was posted on “How To Mosaic.” I used small grout spaces and since I wanted the piece to look like stained glass I used dark grout colors. Since the piece contained 4 panels I used 2 different colors: sable brown on 2 and gray, which was slightly lighter, on the other 2. On my table, before light was shining through it one could discern the two shades. However, once mounted in the window, during daylight hours this difference is non-existent, since the daylight renders all opaque surfaces in silhouette. I would therefore recommend using a dark-color grout, as this creates a close resemblance to traditional stained glass.

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