Mosaic table by Julie Landberg showing grout "stain" in superficial pitting of vitreous glass mosaic tile.

Cleaning Grout from Pits in Vitreous Glass Tile

Vitreous glass tile has a lot fewer surface pits than it did a few decades ago, but it still has some, at least in most brands. Grout can sometimes lodge in these superficial pits if you don’t adequately sponge and haze the mosaic after grouting, and many novices find this problem particularly distressing.

The good news is that the problem can be fixed easily, and it can be avoided for the most part by correctly sponging and hazing the mosaic during the grouting process.

Avoiding The Problem

You do NOT need to find vitreous without pits to avoid the problem of pit staining.

Keep in mind that the same vitreous tile is used as an architectural covering for thousands of square feet at a time, and so common sense tells you that there has to be a way to get grout out of pits without a large amount of labor.

I feel the need to stress that point because I receive emails from beginning mosaic artists who are desperately searching for vitreous glass tile that is perfectly smooth without the occasional pit: You do NOT need to find perfectly smooth tile.

Rather than limit your selection by such an unnecessary constraint (and spending significantly more than you have to), I think it is better to learn basic techniques and how to work with common materials.

But, it takes a few mosaics before a person has a feel for how much rubbing can be done with a barely damp sponge before you strip the grout line and expose grains of bare sand.

Sometimes the AC or central heat or sun or wind dries things out and forces you to grout faster than you would like, and sometimes you are just too worn out after grouting and cleaning up from that to spend enough time rubbing the mosaic with a barely damp sponge. That is why there is a way to fix pit staining when it happens.

Fixing the Problem

Recently artist Julie Landberg emailed us about a problem with grout in the pits of her mosaic table top. The problem was particularly visible because the tile was white and a nice black charcoal grout had been used.

I recommended our usual method of using cotton swabs and vinegar to surgically clean the pits, and I explained how she needed to work quickly and rinse the mosaic thoroughly to avoid the possibility of damaging the grout line. Sometimes a single drop of vinegar will get on the grout line and “bleach” out a spot by eating up the pigmented grout and exposing the lighter colored sand.

Julie emailed me pictures of the cleaned up mosaic, and said that she also used baking soda in the cleaning process by dipping the vinegar swab in the baking soda before using it to scrub the pit.

The baking soda not only neutralizes some of the acetic acid in the vinegar (making it less aggressive) but it also acts as an abrasive. This makes it easier to clean the grout in the pits but less likely to damage the grout line unintentionally.

Julie's mosaic detail after grout in pits was cleaned.
Julie’s mosaic table after cleaning.

A Warning

If you drip vinegar on a chip of marble or a piece grout, you can hear the acetic acid dissolving the calcium carbonate and see the fizzing bubbles of gas escaping, That is how strong vinegar is, and you have to use it quickly to avoid damaging the grout lines in the mosaic. It will take out the good with the bad. Work quickly and rinse thoroughly.

Julie Landberg's finished mosaic table with sun in sky design over field of flowers.
The finished mosaic table top. Julie’s design features a sun in sky surrounded by blooming flowers of mixed type inside a band of green.






14 responses to “Cleaning Grout from Pits in Vitreous Glass Tile”

  1. Caren zane Avatar

    Hi, are you saying to do this technique after your grout has set .Meaning, before the 24 hr. Time frame it needs to completely set? I have always been concerned to use vinegar before complete set up time.

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Caren,
      That is a very good point. I would wait at least 3 days before using vinegar to make sure that the grout in the gaps has hardened enough in case any gets on them accidentally.

  2. Ali Baker Avatar
    Ali Baker

    Thank You, Joe!

  3. Michele Petno Avatar

    I also soak wood tooth-pics in vinegar to pick out residual grout, then wipe immediately with soft dry cloth.

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Michele,

      Toothpicks are a great idea! We also sell dental pics, and I forgot to mention in the article that we also sell dental picks, which are also used for that. I think that the wooden toothpicks might be a better option because they aren’t as likely to accidentally flake off slivers of glass if you pull wrong.


      1. Michele Petno Avatar

        Hi Joe!

        The tooth-pics soften up in the vinegar but are still ‘sharp’ enough to remove the yuk.

        I found out the hard way on removing haze with sulfamic acid (same outcome with vinegar) I did a huge porch floor in broken tile that had a matte finish and grouted it with chocolate colored grout. The matte hung onto the haze so I cleaned with the sulfamic acid. I nearly had a heart attack when it dried, all blotchy and faded. When the owner came home he was amazed that I could “age the grout” – he loved it!! After that I offered the “aged” look and charged extra for it, lol!

        All the best, Michele

  4. Chris Redman Avatar
    Chris Redman

    Thanks for the tip. I have this problem too – and have been avoiding vitreous glass for that reason. I have painted the tiles with grout sealer before I grout – I think it helps a little.

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Chris,

      Grout and tile sealers are invisible pore sealers that fill micro pores but not visible divots and pits. I think they might help because they keep the grout from sticking inside the pit even though they don’t fill it. The important thing is to apply the sealer with a rag that isn’t dripping so that you don’t get the sealer down in the grout gaps where you want it to stick.


  5. Mollie Seymour Avatar
    Mollie Seymour

    how do you send pictures of your work to you for possible inclusion in this blog

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Mollie,

      We like to receive all sorts of photos of people’s mosaic artwork. Sometimes we can use them in the blog if they address a concern or topic that is coming up. Please email your photos to the help email address at the website, and we will take a look. help[at]


  6. Lizette Murray Avatar
    Lizette Murray

    Hello Joe.

    Do you sell sand grout? And how about that red copy paper?

    Thank you, Lizette Murray

    1. Natalija Moss Avatar
      Natalija Moss

      Sorry for late reply, but we don’t sell grout anymore. It is still very cheap at hardware stores though. As of this writing, you can still get a 50 pound bag of thinset for under $15 at some of the big box retailers. Grout will also come in 10 pound bags for under $15 as well.

      We don’t have red paper, but we do have brown mounting paper squares for use with the mounting grids.

  7. Margee Avatar

    I have had problems with black grout sticking in pitted vitreous glass. There is a product called grout release. I am wondering if painting it on the pitted pieces with a cotton swab or painting on grout sealer pre grout is advisable. I don’t want the bright colors to dull.

    1. Joe Moorman Avatar
      Joe Moorman

      Yes, that is what grout release is used for. It will not dull the colors. It is removed following grouting.

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