Tag Archives: troubleshooting

How to Build Up Areas and Fill Holes in Mosaic Backers

For dry indoor mosaics, areas can be built up to support thinner tile next to thicker tile by mixing sand or sawdust to Weldbond adhesive to create a heavy paste. You can also fill holes in mosaic backers using this method. Sand is best for minimizing the contraction that happens as the glue dries, but sawdust can be used when weight is an issue.

For outdoor and wet mosaics, thinset mortar can be mixed with clean pea gravel to make a concrete. The pea gravel must be washed clean and dried to avoid contaminating the mortar, and we recommend screening the pea gravel and using only small pieces. Note that you cannot use sand because too much sand will overwhelm the mortar and make it crumbly.

Case Study: Stained Glass Mosaic Coffee Table

Stained Glass Mosaic Coffee Table

Stained Glass Mosaic Coffee Table by artist Holly LaBarre.

Recently our customer Holly LaBarre emailed us asking how to build up areas between wooden slats of a mosaic coffee table she was making. The design concept for the coffee table was to make it look like it was made from pallet wood or wooden planks and put stained glass mosaic between the planks. The problem was that the planks are significantly thicker than the stained glass, but Holly wanted the stained glass to flush with the planks so that the coffee table had a flat surface.

The solution was to put a piece of 1/2-inch plywood on the underside of the table top and then build up the area to be mosaiced using a paste of Weldbond and sand.

Mosaic backer built up

Mosaic backer built up. This photo shows one trough with an initial filling of paste. The other two troughs are still empty and show the plywood backer underneath the table top.

The catch is how to build up the area to the right height so that when the stained glass is applied, it isn’t too high or low. To figure this out, Holly made a simple gauge from a straight edged piece of wood that rests on the surrounding planks and has a small piece of stained glass glued to its bottom edge.

simple gauge made from straight-edged board

A simple gauge made from a straight-edged board and a piece of stained glass glued to the bottom shows how high the area needs to be built up. This photo shows that more paste needs to be added, probably 1/2 inch more.

 

Matching Grout to a Room’s Color Scheme?

Choosing a grout color is more of a situation where you want to avoid making a mistake that causes the tile to look wrong than it is an opportunity to tie in the room’s color scheme by selecting some optimal color.

A Case Study

matching grout colors

Should you match grout color to a room’s color scheme? Not necessarily. Making sure the grout color works with the tile colors is much more important.

Recently a customer emailed us the photo above and asked for advice on how to match the grout color to the room’s color scheme, which includes rich gunstock brown cabinets and paint that is pale green or taupe and a black counter top. The mosaic backsplash itself is made from long gray and black tile in varying lengths.

Choosing By A Process of Elimination

Grout colors should always contrast tile colors enough so that each tile is visually distinct. If you were to use a gray grout on this mosaic, the gray tiles wouldn’t stand out as individual tiles. If you used black grout, you would would have the same problem with the black tiles. Since the mosaic is a gray and black color element, a white grout of some shade makes sense. A pure white grout is likely to be too bright, and so an off-white grout that is more or less the same color as the exposed backer between the tiles would be a safe choice.

Too Clever for Your Own Good?

What if you still thought that you needed to tie in the grout color to the room’s color scheme? Then you might consider using some sort of terracotta or brown grout in either a light or dark shade. The problem with that approach is that there are many different hues of gray, and not all of these will look good with a particular brown, even if that brown is optimal for the room.

If you are bent on using some sort of brown or other color for a black-white-gray backsplash such as this, then make sure you take some of the tile with you to the building material store and actually hold the tile up to the grout swatch. That way you can see if the hues look odd together. Avoiding that mistake is much more important than trying to match the other colors in the room.

How You Know This Is Good Advice

Notice how the counter top is black, and the stove and microwave oven are black and silver in color. They don’t have any brown or taupe color elements, but they are perfectly at home in the room’s color scheme. Similarly, the mosaic backspash is a black-white-gray color element that needs nothing extra to tie it in.

 

 

Gaudi Mosaic Bench Freeze Damage

A few years ago, Karen J created a mosaic bench in her backyard using mining debris (large stones), cement, and chicken wire to form the base, which is similar the methods we recommend in our instructions for creating bases for outdoor mosaic sculptures. Karen modeled her bench after those made by the great mosaic architect Gaudi in Park Guell in sunny Barcelona, and she used brightly colored ceramic tile just as Gaudi had used. The problem is that Karen’s backyard is in Colorado, and so her mosaic experienced many long and hard freezes that a mosaic in Barcelona would never see.

Mosaic Bench after Antoni Gaudi

Mosaic Bench after Antoni Gaudi shows the ravages of freeze damage. Colorado winters are quite severe, but any temperature below freezing can crack and flake ceramic tile.

Ceramic Tile Is Vulnerable To Freeze Damage

Glass mosaic tile is non-porous, and so water cannot seep in and freeze and crack it, and so glass is preferred for outdoor use, as is porcelain tile for the same reason. On the other hand, ceramic tile tile is very porous and soft, and so water can penetrate it (through tiny cracks in the glazing). Once this water freezes and expands, it cracks the ceramic tile and often causes the face of the tile to flake off.

Mosaic Bench Detail showing freeze damage

Mosaic Bench Detail showing freeze damage. Note that the empty sockets in the blue tile are NOT where tile has popped off. Instead, it is where the faces of the tiles have flaked off due to water freezing and expanding in tiny cracks and pores.

In the photo above, you can see how some colors were more resistant to freeze damage than others. This difference was not due to the color but to the variety of the tile: some brands of ceramic are harder and less porous than others. Also, some brands have thicker glazes, and that can also affect how permeable the tile is to water.

Preventing Freeze Damage

You can minimize freeze damage by sealing your finished mosaic with multiple applications of a tile and grout sealer from your local building material store. Avoid ordering sealers online during winter months because water-based silicone sealers ruin if they freeze during shipment. You should also clean and reseal the mosaic each fall. Small mosaics such as mosaic stepping stones can be brought inside for the winter.

Mosaic Bench Second Detail showing freeze damage.

Mosaic Bench Second Detail showing freeze damage. Imagine how bright the orange and yellow sun was before Freeze Meister blasted it and flaked off the color!

Shrinking Grout: Fixing Holes In Groutlines

Grout does not shrink, but it is prone to forming voids and bubbles if it is not rubbed thoroughly into the grout gaps. These holes are easily repaired.

My friend Fredrik reported a problem while grouting his mosaic portraits of famous rock icons. He described the grout as having shrank in the grout gap.

Shrinking Grout Detail

Shrinking Grout Detail shows voids and bubbles at the intersection of groutlines. This is a classic problem and indicates that the grout was inadequately rubbed and pressed into the gaps.

Real Grout Doesn’t Shrink

The grout didn’t actually shrink, which isn’t possible with traditional grout made from portland cement, sand, and water. (I can’t speak for the newer epoxy-based grouts because we haven’t used them.) What actually happened was that voids were left in the intersections of the groutlines, and these voids got covered with a thin film of grout that then dried out without curing.

Press Grout DOWN Into The Gaps

When you grout a mosaic, it is important to press the grout down between the tiles, and to rub and press the grout thoroughly. Otherwise, voids and bubbles will be left down in the gaps and get covered over with a superficial layer grout similar to how a thin layer of wind-driven snow and ice will sometimes form over a crevasse in a glacier. That is why pressing down is important, and you shouldn’t just rub tangentially across the surface.

You can also “pull” voids into grout by rubbing it repeatedly in the same direction. That is why it is best to rub in circles and to vary the direction of the rubbing randomly.

Avoid Dry Air: Use Humidifiers

Fredrik reported that he wasn’t able to able to spend very long pressing the grout into the gaps because it started to harden almost immediately. It takes grout a while to harden, and this hardening happens by binding water not by drying out, but grout can become prematurely stiff and difficult to manipulate if it starts drying out. Since Fredrik was working in Sweden in February, I suspect the air was very dry due to the heat running, not to mention the already low humidity of the winter air. You can avoid this problem by running a humidifier near your mosaic and by doing the grouting away from heater vents. You can also cover the grouted mosaic with plastic kitchen wrap such as the Saranwrap brand.

How To Fix Holes In A Groutline

The good news is that it is possible to grout voids and holes and bubbles in a mosaic that is already grouted. The only caveat is that the old grout can suck the moisture out of the new grout before it can properly cure and harden. To prevent this, mist the mosaic thoroughly with water before you begin so that the old grout is saturated with water. Note that the mosaic should not be coated with water because droplets of water or a thin layer of water could interfere with intimate bonding. It really helps to have a humidifier running near the mosaic, and you should start the humidifier an hour or so before you grout so that the old grout isn’t bone dry.

Before you regrout, you need to expose all the hidden voids and bubbles. You can do this by pressing on the grout with a small screwdriver and vacuuming out all the loose crumbles. Of course, you need to do this in places that look like there is a problem, and but you also need to press in places that look deceptively fine. You don’t want to have to grout a third time.

I prefer to press straight down instead of dragging the screwdriver because I want to avoid scratching up grout that is fine. If you do get some scratched up grout from your probing, use a stiff bristle brush and a little water on the finished mosaic to buff out the scratches and make them less noticeable.

If you notice that your grout is crumbly in general, you should scrape it all out and regrout. In that case, you might want to use the grout removal tool.

Mixing Up Small Amounts of Grout

One objection to fixing small grout problems is that people don’t want to mix up a whole container of grout and end up wasting almost all of it. The good news is that you don’t have to waste any or at least very much, and you don’t have to be paranoid about whether or not you are adding the right amount of water. You can easily mix up a small amount using a few rules of thumb:

  • If you have a small postal or kitchen scale, use 1 part water by weight to every 4 parts dry grout.
  • If you don’t have a small scale, add a little water and mix thoroughly. Stop adding water when the grout has the consistency of dough.
  • Once you have enough water in the grout, mix it thoroughly to ensure even consistency with no tiny lumps of dry material. These can sabotage the grout hardening process.

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.