Tag Archives: thinset mortar

Mosaic Concrete Lawn Sculptures

There are two options for bases for making a mosaic sculpture for your lawn:

  1. Buy an unfinished concrete sculpture from a lawn and garden center or a store that specializes in concrete lawn sculpture.
  2. Make your own concrete sculpture using cement and chicken wire and pea gravel and similar reinforcing materials as discussed on various websites found by searching Google for “concrete sculpture” or our page for how to make concrete mosaic sculptures.

Note that my page focuses on a large structural base I was making at the time while some of the other websites have better pictures of how to crumple up chicken wire to make smaller figurines. However, the Internet contains a lot of problematic advice about hypertufa and other practices, so make sure you read my caveats below.

Each of these options has advantages and pitfalls that are important to consider before deciding which route you will go. Either way, following best practices to prevent moisture penetration is critical for ensuring the longevity of your mosaic lawn art.

Frog mosaic lawn sculpture

Frog mosaic lawn sculpture by artist Lyn Richards is an excellent example of how you can take a mass-produced concrete base and make it your own unique piece of art by how you tile it. Note how the artist used multiple green hues instead of one monolithic color field and how the greens contrast the reds and yellows, which are arranged in patterns that suggest motion.

Purchasing A Factory-Made Base

A factory-made base purchased from a lawn and garden center will be identical to all the other pieces made from the same mold, but there is obvious the advantage that you can start mosaicing immediately without having to make the base yourself, which is a complete project in itself and tends to be more labor intensive than expected, unless the artist has worked with concrete before. Also, you can make the generic base into an original work of art by how you tile it, as demonstrated by the artwork by Lyn Richards featured on this page.

Sealants Can Interfere With Bonding

If a drop of water soaks into the surface of the concrete, there is no excess sealant present, and the thinset mortar used to attach tiles should be able to bond securely.

Sometimes you can find factory-made sculptures that have been drenched in sealants, and these can interfere with thinset’s ability to bond to the concrete securely, and tiles could fall off over time. If a drop of water beads on the surface of the sculpture (similar to how water beads on a waxed car), then you know the sculpture is coated in a sealant that should be removed before mosaicing. To remove the sealant, scour the surface with a stiff wire brush of the type used to clean welds (the wires are stiffer and more coarse than those on most wire brushes used to clean barbeque grills).

Butterfly mosaic planter

Butterfly mosaic planter by artist Lyn Richards was made on a concrete base, which are much more durable than terracotta planters. Note how the artist used the grout line to add even more detail to the butterfly’s wing. Also note how the black grout makes the colors look even brighter.

Beware Of Lightweight Concrete

Sometimes factories will mix expanded perlite or other materials into the concrete to make it weigh less than regular concrete. This is fine in theory, but the expanded perlite is highly porous and mostly air and therefore readily absorbs water. This makes the sculpture highly vulnerable to freeze damage: water seeps in, freezes and expands, and then the surface of the concrete flakes off. If you find a sculpture that appears to be lighter than expected or has a softer or more porous look to it, then coat it all over (bottom included) with a layer of thinset mortar and allow that to harden before you mosaic on it.

mosaic bird bath

Birdbath mosaic by artist Lyn Richards. Outdoor mosaics on horizontal surfaces tend to have shorter lives as standing water penetrates between the tiles. Mosaic benches and birdbaths and other horizontal surfaces should be sealed with multiple applications of a tile and grout sealer to prevent this from happening. You should also consider cleaning and sealing these surfaces every year or two.

Making Your Own Concrete Sculpture For A Base

Mixing up even 5 pounds of concrete can be labor intensive, and mosaic lawn sculptures of any size at all can require 25 to 50 pounds at least. As you might recall from mixing up grout, your arms feel it even with the small batches, and most people will find a drill with mixing paddle necessary to mix up anything over 10 pounds.

Several of the websites explaining how to make concrete sculpture emphasize the need to make the bases hollow or to use Styrofoam or harder varieties of expanded polystyrene to fill internal voids. This is good sound advice, especially if you make the surrounding shell of concrete structurally sound by reinforcing it with metal rods and wire.

Thinset Instead Of Regular Concrete

There are different concrete mixtures recommended on the various websites, some of which include glass fibers used to give the concrete the tensile strength it normally lacks. For my concrete sculptures, I used thinset mortar instead of regular concrete. Thinset mortar is like regular cement with sand, but it also has polymers for adhesion and tensile strength. I also mixed in fine pea gravel for additional strength and bulk. The pea gravel was sieved to remove larger pieces and added 1 part pea gravel to 2 parts thinset on a weight basis.

Thinset is more expensive than regular concrete, but it is so much stronger, and you don’t have to worry about whether or not it is adhering well to a bare wire frame. Also, if you have to add concrete to a large frame in multiple batches mixed in multiple studio sessions, you can be confident that the fresh thinset is adhering to the thinset that is already hard, and you can’t say that about regular concrete, which is likely to form a crack at the boundary.

Of course, the concrete sculpture recipes I saw on various websites recommended different latex additives that should be added to the regular concrete to make it be more like thinset. But my thoughts were why reinvent thinset from different components if I could simply buy thinset, which was actually cheaper by the time you factor in the additives. Also, instead of picking or sieving the larger rocks from a bag of concrete, I thought it made more sense to start with a bag of thinset (which contains no stones) and add the exact size of pea gravel I needed to ensure it fit into my wire mesh frame.

Hypertufa

Hypertufa is synthetic version of a type of porous limestone called tufa, and it is popular for making custom planters because its porosity is good for plant roots and getting covered with moss and lichens. Hypertufa is made by mixing peat moss and perlite and sometimes other materials into concrete to make it porous and lightweight.

About half the post you see on gardening blogs repeat the mantra the hypertufa is completely freeze proof. The other half are asking for advice of how to prevent their hypertufa planters from mysteriously cracking (usually with an acknowledgment that they accept the official dogma that the cracking couldn’t possibly be due to freezing temperatures).

What I do know is that all other porous materials I have encountered in my experience with outdoor mosaics are highly susceptible to freeze damage: terracotta, unglazed ceramic, unpolished stone, etc. If it has small holes in it, even tiny microscopic holes, then moisture penetrates and freezes and cracks or flakes the surface.

I’m not sure why hypertufa would be any different. The peat moss and perlite are ready conduits for deep moisture penetration. I strongly suspect that hypertufa is so soft that the damage due to freezing and expanding doesn’t result in macro cracks right away, and thus people with weak reasoning skills assume it is freeze proof. Later when cumulative damage finally results in a crack, the crack is attributed to the anger of the hypertufa gods or something like that.

Fish mosaic planter

Fish mosaic planter by artist Lyn Richards makes excellent use of glass gems as bubbles. Notice the mixed use of green hues in the seaweed and how the fish and weed are integrated and how they make use of contrasting colors from opposite sides of the color wheel (green and orange). The white makes these colors “pop” even more, as does the black grout.

I strongly suspect that hypertufa is so soft that the damage due to freezing and expanding doesn’t result in macro cracks right away, and thus people with weak reasoning skills assume it is freeze proof. Later when cumulative damage finally results in a crack, the crack is attributed to the anger of the hypertufa gods or something like that. “I must have done something wrong in how I mixed up the hypertufa or cured it. It couldn’t be the freezing winter conditions.”

All that being said, hypertufa is fine for someone making a simple un-mosaiced planter and wants it to crumble away slowly like natural limestone. But if you are going to the time and expense of covering it with mosaic tile, then avoid hypertufa. If you want to make a lighter core from concrete mixed with perlite, that is fine, but cover the outside with a layer of thinset before you mosaic to ensure that the vulnerable porous material is protected from the risk of moisture penetration and freeze damage.

How To Mosaic A Concrete Lawn Sculpture

Like all outdoor mosaic and wet mosaic, the tile should be attached with thinset mortar instead of glue. Other than that, the instructions are more or less the same as our instructions for regular flat panel mosaics. I wrote some detailed instructions for using thinset mortar for mosaic art.

You can grout an outdoor mosaic with thinset instead of grout. Thinset is harder and more water-resistant than traditional grout, so it is probably better to use thinset for grouting. Thinset can be dyed with concrete dye. (Note that I am always talking about traditional powered thinset mixed with water and NOT the new epoxy-based systems with a liquid component). I use Versabond thinset in my work, and I have exceeded the dye manufacturer’s maximum recommended amount of dye in the thinset by a factor of 2 without affecting hardness or bond strength in any way that I could notice, but I am sure that it is possible to add too much dye depending on the brand.

Tip: Put cardboard on your work table to protect it from the weight and roughness of the concrete sculpture.

Tip: Rest the bottom of the sculpture on small blocks such as stone tiles or whatever so that you can mosaic and grout the bottom edge of the sculpture without the surface of the table getting in the way.

Tip: Consider sitting your sculpture on some plain concrete stepping stones when you install it in the garden by raising it slightly off the moist soil, you can greatly increase the life of the mosaic, especially the tiles near the bottom edge.

Tip: Use a pencil or marker to draw patterns on the surface of the sculpture.

Tip: Never use one color blue to make a color field when you could mix two similar blue colors to make the same field of color. It makes the element more visually interesting.

REQUIRED: After your grout has hardened for a few days, seal your finished mosaic sculpture with multiple applications of a tile and grout sealer following the instructions on the package. It wipes right off glass tile and only seals invisible pores.

Thinset On Plywood Mosaic Backer?

Thinset mortar can be used on a plywood mosaic backer provided the plywood has been sealed with a white PVA adhesive such as Weldbond. Otherwise, the plywood can suck the moisture out of the thinset before it has a chance to harden leaving it soft and crumbly. It is important to seal the plywood at least a day or two before you mount the mosaic with thinset because the Weldbond needs to be thoroughly dry and cured. For sealing purposes, Weldbond can be painted on with a brush or spread with a trowel. Many mosaic artists dilute the glue slightly or even up to 1:1 with water to make it easier to spread. Note that you shouldn’t seal your plywood with paint or sealers chosen at random because thinset will not bond to some of the oil-based and silicone-based products.

Some artists seal their plywood backers with Weldbond before mosaicing even if they are going to use that same Weldbond to mount the tiles. Why? They want to make sure that the entire surface of the plywood has been sealed so that it doesn’t suck the moisture out of the GROUT when it cures. I never worry about sealing the face of my plywood because I am sure that I will get enough Weldbond spread around when I mount my tile, and so pre-sealing seems like an unnecessary extra step to me.

Sealing VS Sealing

A note of clarification: In this article, I am talking about pre-sealing the face of the plywood with Weldbond or some other PVA adhesive. This is a different step from sealing the finished mosaic a few days after grouting with a tile and grout sealer or “sealing” the edges and back of the plywood with paint or varnish to prevent warping by moisture.

When You Should and Should NOT Use Plywood

Plywood can warp and delaminate over time merely from the humidity in the air, and so you should never use plywood as a backer for outdoor mosaic or a mosaic in a damp location. However, plywood makes a great backer for dry indoor mosaics and not just for its light weight, which in itself is a significant advantage over concrete backer board. Unlike concrete backer board, the edges of plywood can be stained or painted, and they do not shed crumbs of concrete and sand. In my opinion, concrete backer board should only be used inside walls and floors and other places where its crumbly edges are covered up; it is not a backer for moveable mosaic plaques.

Use Cabinet-Grade Plywood

Note that you should spend the extra money and buy cabinet-grade plywood (in 1/2 inch thickness) instead of the ordinary plywood used for sheathing in construction. The cabinet-grade stuff comes with a sanded finish and has no knots or internal voids. It is also more resistant to warping, and the edges of cut pieces are cleaner than those cut from regular plywood mainly due to the lack of voids in the internal plies. With all of these advantages, the cabinet-grade plywood is only about 15% to 25% more than regular plywood, and in real terms it is actually the same price: some of the regular plywood may need to be scrapped due to internal voids, and it will require more work to make the edges presentable. The edges of a cut piece of cabinet grade plywood can be left plain or painted. The edges of a cut piece of regular plywood will have voids and require putty, sanding and paint if not a frame to look presentable.

Why Use Thinset on Plywood?

If plywood can only be used for dry indoor mosaics, then why would someone need to use thinset mortar on a piece of plywood anyway? The simple answer is there isn’t any reason to do so (IF you are using regular flat tile). With flat glass tile, you simply glue the tile on with Weldbond or some other PVA adhesive, let it dry for a day or so and then grout it by rubbing wet grout into the gaps and wiping away the excess.

However, some people (including myself) like to use rounded and irregularly-shaped found objects in our mosaics, and these are not easily grouted. Instead of the glue-then-grout method, we like to press our objects into a bed of mortar so that the excess mortar presses up slightly around the object and skip grouting entirely. It is also necessary to avoid grouting if your found objects are naturally porous (seashells, bones, unpolished stones) because grout will stain these materials.

solstice-door mosaic detail

The face of “Solstice Door” mosaic stele was made from a lifetime’s collection of found artifacts embedded in thinset mortar. Note that the stele is a reinforced concrete monument. Plywood would not support the weight of these materials.

How To Use Found Objects In Mosaic Art

Making and grouting a mosaic from ordinary flat tile is relatively straight forward. However, adding three-dimensional objects such as porcelain figurines or even simple rounded stones can complicate the grouting process. Instead of using a grout float or damp sponge to wipe away the excess grout, you may need to use a gloved hand as we often do, and you may have “excavate” some objects using brushes or dental picks if they have very complex shapes with nooks and crevices. An old toothbrush can be useful for cleaning wet grout from crevices in objects, but note that brushes of all type remove grout very aggressively, so take care that the toothbrush doesn’t gouge out any grout from where it needs to be.

If your mosaic is mostly dimensional objects, then the amount of labor required to remove excess wet grout may be excessive, and the easiest way to solve the problem is to avoid the grouting process entirely. Also, unsealed porous objects such as seashells cannot be grouted without staining them with the grout.

For these situations, it is better to press the objects into thinset mortar instead of the usual glue-the-grout method used for making indoor mosaics.

Thinset is a type of concrete, so if you spread it on slightly thicker, then a little bit will press up around the sides of the objects and serve as grout between the tesserae (“tile”). The key is to not spread so much thinset so that the object sinks in too deeply or too much thinset squeezes up and around the object.

Before you start pressing your childhood treasures and precious souvenirs into the sticky wet concrete that is thinset, you will want to practice for a few minutes with some ordinary rocks on piece of scrap. That way you can figure out how thickly to spread the thinset to avoid problems and practice keeping your hands clean while working with wet concrete. Otherwise your fingertips are likely to get wet concrete on objects that aren’t easily cleaned.

Your quick experiment on a piece of scrap should use more than one size of rock and include pressing objects close together. That way you can decide how closely you want to space your found objects in your actual mosaic and see how small objects have the potential to be buried in thinset pressed out by large objects nearby.

Keep in mind that small experiments like this do not require any extra time. Whatever time that is required to do them is almost always made up for in the time they save you on your project.

 

How To Build Up Low Areas With Thinset Mortar

Low areas on a surface can be built up with thinset mortar mixed with an additional aggregate of very coarse sand a few days before tile is laid. This should be be done if you want to put a mosaic insert of thin glass tile (1/8 inch) between thicker architectural tiles like 3/8 inch stone and ceramic and still have a flush surface.

A Separate Step Is Best.

This must be done before you mosaic, preferably a few days before or even earlier. Remember, thinset shrinks or thins as it cures. You should not try to tile and build up areas at the same time.

How to Minimize Thinning During Curing

The idea is to add coarse sand with the fine stuff sifted out. (This is in addition to the fine sand aready in the thinset.) The best aggregate is the smallest grade of crushed stone. You want the largest piece size that is practical. For example, if you are building a layer 3/8 inch thick, you might not want a grit larger than 1/16 inch. If the grit is too large, you might have trouble making a smooth level surface that doesn’t have pieces of grit sticking up too high.

Make sure you save about 20% of your thinset batch free from coarse sand. You will need some plain thinset for wetting surfaces before applying the thinset with aggregate added. Scoop this into a separate container after you mix up your thinset.

How To Sift Sand Without a Mesh Sieve

Of course your sand has to have some coarse stuff in it for this to work. Bags of sand from the concrete aisle of the building material store won’t work for this because it is already sieved to be fairly small and uniform.

Colander or Two Buckets

If the sand is coarse enough, you may be able to wash it in a colander. If it is smaller than the holes of your colander, then you can use two buckets.

Do It Outside With a Garden Hose

In either case, do it outside with a garden hose somewhere water and sand can slosh. Sand should never go down drains.

2-Bucket Method

Put the sand in a 2-gallon plastic bucket. Fill the bucket 1/3 the way up with sand. Fill the rest of the way up with water. Swirl the sand with with a large serving spoon (Use a one-piece spoon. Don’t use welded ones. The scoop tends to break off the rod eventually.)

Allow the sand to start to settle, but pour it off when a sand slurry is still spinning around in the bucket.

Pour it off in a neat flowing motion that doesn’t jerk or jolt the bucket or otherwise prevent heavier particles from settling out. Don’t pour off the sand in the bottom.

A 5-gallon bucket is a good size for the bucket being poured into. Allow the 5-gallon bucket to settle for a few minutes and pour off the water in it.

Add more water to the sand still in the original 2-gallon bucket and repeat the operation a few more times or as many times as needed.

Start With Mostly Coarse Sand

It really helps if you start with sand taken from a place where the sand is already the coarsest. Look at the pile at the garden center or creek bed and pay attention to where the water has washed the smaller pieces away leaving mostly the good stuff.

Pick out random rocks and pieces that are too large.

Mixing In The Coarse Sand

The sand needs to be dry before you mix it into the thinset, else you are adding water to your thinset.

Add no more than 50% coarse sand, the coarser the better. If your sand is too fine, it will overload the thinset because the small stuff has a lot more surface area to wet. Make sure you don’t add more sand than the thinset can coat. Add some sand and mix it in before mixing it all in. Stop as the mixture becomes saturated. The coarse grains need to look like something breaded for frying.

You can also so this with small pea gravel to make 3-D concrete figures for mosaic sculpture.

Applying To The Surface

Wet the surface with thinset before troweling on the thinset/coarse sand aggregate that you mixed up.

Keep in mind that it will still contract some if the low spot has any depth to it. The coarser the sand you use, the less it will contract. It is possible to make the bulk contraction small enough
to not be noticeable.