Mosaic Pizza Oven

Mosaic Pizza Oven

Mosaic Fireplace and Oven Surrounds: The Basics

A couple of years ago, I wrote a page explaining how glass, ceramic, and stone tiles can be used for mosaic fireplace surrounds and how the tiles should be mounted with thinset mortar or white PVA (polyvinyl acetate) adhesives such as Weldbond. But we are talking about the SURROUNDS, not inside the firebox. For inside the firebox, your need to use refractory materials (brick or stone) that can resist combustion temperatures. For the hearth, the issue is not temperature resistance so much as impact resistance: It doesn’t make sense to use glass tiles that are easily cracked by a metal poker or small tiles that are easily knocked loose. Stylistic concerns should never outweigh performance and durability, else the work won’t look good for long.

Mosaic Pizza Oven
Mosaic Pizza Oven by artist Kristina Young with octopus tentacle motif. Seafood and sea life and undersea scenes were common themes of Roman mosaic.

Problems with a Mosaic Pizza Oven

Recently, artist Kristina Young emailed me concerning a problem she was having with a mosaic she installed on the outer surface of an Italian pizza oven. The problem was that the mosaic was cracking over the door of the oven, and that caused me some concern because that should not happen with traditional fireplaces and pizza ovens constructed with brick or stone, and I have been telling people for years that there was no reason why they could not put mosaics on these surfaces in spite of the heat. Had I overlooked some basic technical principle and made recommendations that could ruin hundreds of people’s projects? The engineer in me became completely paranoid, and I could not wait for Kristina to email me back with answers to my initial questions.

Spoiler Alert: The good news is that the cracking is reparable and that the cracking is by the iron frame of the oven door, not the masonry elements of the oven itself, which means that there is no reason to expect similar problems with traditional fireplaces and ovens that are made from all stone or brick or concrete.

The Case of the Cracking Mosaic

cracking mosaic detail
Detail shot showing crack in brand new mosaic covering the exterior of masonry pizza oven. The location of this crack is significant: It started  right above the iron frame of the oven door.

When Kristina first contacted me, she was concerned that the cracking might have been caused by heating the oven not long after the mosaic was completed. That is a potential issue because thinset mortar takes time to harden, and like concrete, it hardens by bonding moisture not by drying out. (Concrete, mortars, grouts, and other portland cement products will be soft and crumbly if they are dried out by heat or dried air. They need to incorporate the water mixed into them, not have it removed artificially.)

Humidify, Don’t Heat

I don’t think that the oven was heated prematurely or that premature heating caused the cracking. The crack is location specific, and if the mortar was artificially dried out before it could harden, then the problem would be seen all over the mosaic in the form of cracks and missing tiles. That being said, I would avoid heating fireplaces and ovens for several days after a mosaic has been applied to them and grouted. The usual practice is to run humidifiers near a new mosaic to protect them from AC or central heat –not build a fire under them!

Thermal Expansion

Except for the notable exception of ice, most materials expand when they are heated. (Water expands when it freezes, and that is why ice floats: it is less dense than the water beneath it.) The problem with thermal expansion is that materials expand at different rates, and metals like iron expand more rapidly than stone, brick, and concrete. Kristina had already told me that the crack started on the front of the oven just over the door, and so as soon as she sent me a picture of the oven showing that the door had an iron frame, it was obvious to me why the crack had started there: The glass and mortar mosaic expands at roughly the same rate as the brick and concrete oven underneath it, but the iron door frame and the other iron structural elements expand even faster. They push the mosaic up like a shell on the outside of the oven, and when the oven and frame cool back down and contract, the crack appears.

The Right Repair Materials

An “expansion joint” spontaneously forming in the middle of your mosaic might have most people panicking and thinking of repairing the crack with a flexible material such as caulk. Caulk is problematic because it will not age well. It will yellow and shrink and crack. It will look more and more like the synthetic material that it is, a material that looks out of place on tile, a material which does not age.

Grout could be used to fill the crack. After all, grout is the concrete product that is used to grout gaps between tiles in the first place. However, thinset mortar is a better choice because it is harder and tougher and more adhering than grout,, and it can tolerate slight displacement (movement) while grout cannot. In fact, it would have been best if the entire mosaic had been “grouted” with thinset. I suspect that heating and cooling the oven in cycles over time may cause other cracks to appear or reappear, and these should have thinset rubbed into them as needed. Hopefully any new cracks or reappearing cracks will be smaller, but in any case, thinset is better equipped to withstand the stresses of expanding and contracting than grout.

Mosaic crack repaired with thinset mortar
Mosaic crack being repaired with thinset mortar. The mortar is spread on and worked into the cracks and wiped off just like grout. Thinset is superior to grout because it is harder and tougher and can tolerate slight movement.

Aesthetics and Authenticity

Think of high-end restaurants in reclaimed urban warehouse spaces: the exposed beams, the plaster chipped away in places to reveal the stone walls underneath, the different architectural elements like fire doors and hoists deliberately left in place to call attention to the space’s past industrial use.

To me, one of the more interesting things you can see in the mosaics of Mexico and the Mediterranean basin are the repairs that have been made to these over the years following earthquakes and other damage. I’m not thinking of the repairs that were made in modern times by archaeologists or professional conservators sparing no expense to make the mosaic look as if the damage had never occurred. I’m thinking of repairs made in the distant past by inexpert hands or by people with limited access to materials. I’m thinking of repairs like mortar-filled voids and replacement tiles of not-quite-the-right color and how you can sometimes see a series of these inexact repairs apparently made at different times in response to different injuries. To me, these inexact repairs more than anything else give me a sense of how ancient the mosaics are and how much history they have witnessed, endured even: earthquakes, fires, wars with slings and arrows, wars with bullets and bombs.

A large part of the ethos of mosaic art is it being an enduring relic of the past. If I were wanting to design a mosaic to look like an old relic, I might consider deliberately including mortar-filled voids and cracks to simulate past damage or maybe re-mosaicing some of these regions with coarser tile. With that in mind, is a crack appearing in a new mosaic in an Italian or Mexican restaurant a problem or a windfall? I’m thinking not. I’m thinking of the kid who deliberately scuffs up his new baseball glove so that it doesn’t look the unused glove of a rookie.




42 responses to “Mosaic Pizza Oven”

  1. Thanks for the article.
    I am making small mosaic heart ornaments for a class I lead and it is called Beauty in Brokenness. I teach as the participants mosaic and learn the analogies. I want to speed up the process and am considering using thinset as both the adhesive and grout by using a thicker amount on the medium and letting it squish up between the tile pieces. Once pieces are laid, could I then use a barely damp sponge to wipe the surface and the tiles/china/porcelain will come clean and be like grout?
    Hoping to do a quick project in an hour or so from start to finish with folks who may never have done mosaic before. Open for suggestions! Using adhesive piece by piece and then letting it dry then grouting with sanded grout is real messy and time consuming. Loved that you mention using Thinset for both but I’ve never used it!

    • Amy,
      My apologies for not replying sooner. I was tied up on a project and only monitoring email. That is the best way to reach me for urgent questions. I do what you proposing all the time and press tile into thinset and not grout later. However, that approach requires that your tile spacing be a little bit “looser” (more widely spaced) than the glue-then-grout approach. Another thing to consider is that thinset is like sticky grout (because it is essentially grout with adhesive added), and so it isn’t exactly mess free. The benefit is that you can do everything in one session. I hope this helps!

  2. Hi Joe,
    Thank you for including my mosaic pizza oven as a test case on your blog! I hadn’t seen it until today, but am getting back to my website as I begin work on another project.
    Thank you also for your technical support for the oven! I can report that Athena the octopus is doing well and has suffered no more cracks since patching it last fall.

  3. I’m trying to mosaic an outdoor pizza oven in Arizona which is open to the elements of sun, rain etc. I was wondering what people recommend from Mortar to specific grout compound and also what tile works best.

    • I think people use ordinary thinset mortar. We use Versabond from Home Depot for all our work including fireplace surrounds.
      I hope this helps!

  4. I live in Brooklyn, Pretoria. We have a domestic Italian pizza oven at home. i want to mosaic the outer surface of the pizza oven and require someone experienced and professional to do so. Can you assist?

  5. Hi
    I just bought my sweetie a pizza oven and would like to apply mosaic tile on the outside; can yu recommend a site to provide step by step instructions? Also, I am wanting to put it on the inside wall(concrete) of our Lanai. It will be under our patio which is wood. Is there a fire wall plate or something else available to put between the wall and my ceiling?

    • Stone or brick pizza ovens can be mosaiced. Steel ovens cannot because they expand and contract too much with the heat. There isn’t anything particular to the mosaic process that would make it any different from a normal outdoor mosaic mounted with thinset.

      I hope this helps!

  6. Hi,

    Thank you for your blog! It’s really helpful. I want to mosaic an outdoor forno bravo pizza oven in Colorado — so I need to account for wild temperature extremes. I’m hoping to do lots of bright colors, but am wondering the best kind of tile for such an environment. Do you have recommendations? Also, do you recommend doing it directly on to the oven or using a mesh?

    • Glass or Porcelain is best for outdoors. Avoid glazed ceramic tile and marble when freeze damage is a risk.

  7. Please could you tell me how to weather seal the mosaic on an outdoor pizza oven. Is thinset waterproof or do you need to apply something else?

  8. Hi. I know you said multiple times about using thinset to apply tiles and grout them I just want to make 100% sure this is heat proof as our oven is just a little home made one and space was an issue so the insulation layer isn’t as thick as it ideally needs to be and so therefore the outside does get quite hot, will thinset still be ok and a normal tile and grout sealer?????
    Thank you

    • Hi Laura,
      I suppose it would be possible for an oven to get so hot that it degraded the sticky polymers over time, but it’s impossible to say from a general description. I wouldn’t think it would be an immediate problem because concrete isn’t degraded rapidly by oven temperatures, and the corrosion is on the inside.

      But first you needed to makes sure you are not creating a fire hazard or structural problem or a code violation with the oven as it is.

  9. Hi
    I love in the North East of England in the UK, we have an outdoor pizza oven, which I am looking to Mosaic tile.
    Reading through your replies to various comments, it looks like glass or porcelain tiles are best to use, and to use a thinset mortar to bond and grout the tiles. Then apply a tile and grout sealer to seal the grout.
    Would you allow the mortar to dry out prior to applying the sealer, if so, roughly how long would I need to leave it to dry out.
    Buying sheets of mosaic are expensive, would it be ok to buy full tiles, break them and make a random mosaic with them.

    • Hi Neil,

      Mortars and grouts harden by binding water instead of drying and so you actually shouldn’t do anything to accelerate drying. I usually let a grouted mosaic cure for 2 or 3 days before applying a grout sealer. Many people use broken tiles instead of buying small mosaic tiles.


      • Hi
        I have one last question.
        The outer surface of the pizza oven has previously been rendered, then painted over with masonry paint, will I be able put the thin set mortar straight onto the painted surface, or will I need to add some sort of PVA coating ?

        • Hi Neil,
          I have seen contractor forums where the advice is to just scuff and gouge the paint, and then the thinset should be able to grip well enough, but I am sure that shortens the life of the mosaic, even if the problems don’t happen for 10 or 15 years. BUT, this is a pizza oven subject to thermal expansion and contraction, and so I think the thinset needs to bond intimately to a masonry surface to ensure that it will last. I would want to sandblast the paint off, especially if it were old.

          By “rendered,” I think that means plastered? I would tap it in a few places to chip some off and see how well it is bonded to the structure underneath. Some people do a poor job plastering or use the wrong material or let it dry out as it is hardening, and so sometimes fascia plaster will flake off in big chunks. I could see this being an issue in a situation with contraction and expansion, but if the chip test goes well, it should be fine.

          I hope this helps!

  10. I recently purchased a pizza oven from a retired Italian guy from the motherland, were talking an old school gentleman who appreciates his hand built ovens. Unfortunately he doesn’t tile the exterior and leaves it a simple mortar shell. The outside of the oven does get warm, but I can rest my hand on it comfortably when the fire box is at 800 degrees. Should I use refractory to set tile or would a standard thinset be ok.

    • Unfortunately, a metal oven would expand and contract at a different rate from the mosaic and mortar and crack it off.
      Sorry about that.

  11. Hi, i have a Mobile wood fire oven (stone) and I want to cover it with mosaics. What type of mosaics and mortar you recommend ? I drive my trailer with the oven to different events I just want to make sure that mosaics will resist the movement.
    Thank you

    • Hi Cesar,
      Any mosaic made with thinset should be as resistant to vibration as the stone oven it is mounted on. As an engineer, I am concerned about the stone oven itself. What kind of mortar was used in its construction? How much has it been moved around? I would make sure that I had moved it around for some months to see if it were resistant to cracking before I invested the time and money required for a mosaic.
      Please let me know.

  12. hi I don’t think w have thinset in Australia do you know what glue to use here or just a tile glue

    • Hi Chris,

      LOL. That’s probably like saying they don’t have bricks or plywood in Australia. They probably call it something else, maybe tile mortar I’m guessing. It looks like concrete without the rocks when in the powder form and after you mix in the water, only it is a lot more sticky. If you go to (or email) a building material store and describe that, they should be able to show you something that fits that description. I don’t like the outdoor tile adhesives. From what I have seen, they degrade and fail over the years.

      I hope this helps more than it frustrates.

  13. You kind sir ! I am thankful I came across your video/ site as I begin the process of building an outdoor pizza oven. Being a potter , my main desire was to have something to tile with tiles that I had made 15 years or more ago . Reading and watching what you have here is very informative . I’ve noted that you are patiently answering many question on here that the video clip answered . Watching the clip answered all my questions , and by watching it I learned much I didn’t know that I needed to know. Thanks again!

  14. Hello, I have a Portuguese bee hive style clay oven. It has some cracks outside. The clay shell inside has very little cracks but they seem to still seal well. I have been researching and I have not seen anyone putting ceramic tiles on a clay oven. What would you recommend to fill the cracks outside and is there a way to water proof it? In was thinking about lime plastering which would add another insulation layer also. Thanks!

  15. Hello Joe, ive stumbled across your threads.
    I’m a tiler and have o job on an outdoor clay pizza oven, its to be tiled in 2″ porcelain mosaic, im having trouble finding a suitable adhesive and grout for this job, mainly due to the heat.
    Any suggestions and help would be greatly appreciated


    • Hi Dean,
      They make a high-temp thinset mortars, but most of the mosaic pizza ovens I have seen use regular thinset.

  16. We bought a house and inherited a pizza oven made of cast concrete about 3-4cm thick. It has a few superficial cracks and has a sort of gauze covering most of it, some of which is coming off. I have some offcuts from Morrocan Zelige (clay) glazed tiles. I was wondering the best way to attaach them to the oven. I was thinking using a fireproof cement to help stabilise it incase the heat crack the concret inside. Would this be a good idea or should I simply use thinset mortar?

  17. Question: can you tile over a perlite pizza oven. If yes, what type of thin set and grout would be used. Does the thin set and grout need to be flexible? Thanks for any help

  18. Hi there, I sound really appreciate your expertise. We have a clay pizza oven, and id like to mosaic it with cracked small porcelain tiles. I was going to render it, these use heat adhesive and grout.
    Do you know if this would effect the integrity of the actual oven itself? I’m nervous the heat would get trapped and crack the clay oven.
    Thank you so much

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.