Thinset and grout

  • Thinset and grout have been used by mosaicists and tile masons for 2500 years. This post is about :
  • what they are,
  • when and why use them.

Thinset versus Grout

Thinset and grout are Portland cement based materials with different purposes :

  • Thinset is a structural material. It bonds your tiles or tesserae to their support, it sets slowly, and is very strong.

  • Grout is a filling material. It flows into the gaps between the tesserae and fills them up to make your mosaic look better. It sets quickly and is not very strong.

Thinset and grout - thinset sticks, grout flows

It is important to remember that while thinset is structural, grout is not. So while there are cases where you can use thinset instead of grout – I will address this further down – you should never use grout instead of thinset.


Thinset – thinset mortar – mortar

These three terms are used by different authors and you might wonder what the differences are. There are none. They are interchangeable. While brick and stone masons commonly speak of  “mortar”, mosaicists and tile masons tend to use “thinset”.

What is thinset mortar?

Thinset is a bonding material. It is a Portland cement (the same as used in concrete) in which polymers have often been added to make it stronger. It is used to make tiles stick to their support or base. Because it has great structural strength, it sets – or cures – slowly. I let my mosaic sit for at least 48 hours before I move them after thinset application.

Which kind of thinset mortar?

I purchase my thinsets from DIY construction materials store (Lowes, Home Depot). I make sure they contains acrylic polymers. I only purchase small (25lb) bags.

a bag of thinset mortar
Acrylic fortified thinset mortar

Which color thinset?

Thinset comes in gray and white colors. For some unknown reason the gray thinset is stronger and lasts longer. I have had several problems with white thinsets that I never had with gray ones. I now exclusively use gray thinset. In addition to being a better product it is also cheaper…

Storage of Thinset.

Thinset does not last for ever. It quickly looses its stickiness, specially in wet climate like we have in Alabama. I purchase mine in small 25 lb bags and store them inside 5 gallons bucket with lids. I never keep a bag of thinset longer than 2 month. Trying to save a few dollars by using old thinset is not worth seeing your mosaic fall to pieces when it turns to dust…

plastic storage bucket.
plastic bucket for storage of thinset and grout.


Avoid breathing thinset and grout dust. Avoid making big clouds when you mix them. If you do, just get out of the room until it settles down.

Cement based materials are mildly caustic, meaning they will slightly burn the outer layer of your skin, so try to avoid contact on your skin, which is not easy unless you wear rubber gloves. But of course, it is not easy to work with gloves… when I mix grout or thinset I try to wash my hands with clean water and dry them up often. Generally my skin gets a little dry after I manipulate thinset or grout. Olive oil usually takes care of this.


What is Grout ?

Grout is a filling material. It is a Portland cement (the same as used in concrete) in which polymers have often been added to make it stronger. Grout has a low structural strength and does not stick to surfaces as well as thinset. It sets quickly (10 to 30 minutes) and needs to be cleaned up shortly after use.

The Use of Grout

Grout is mostly cosmetic, it is a filling material that flows between the tiles and sets there. It makes the whole mosaic look better, and easier to clean up.

Which kind of Grout ?

Grout comes in 2 main categories :

  • Sanded.

  • Unsanded.

Sanded grout should never be used for joints smaller than 1/8“ (~ 3mm). It is thicker and more durable than unsanded grout, but the sand in it can damage smooth tiles by abrasion.

Gap size

Below 1/8” (3 mm)

Above 1/8” (3 mm)




bag of unsaded grout
Bag of grout

Coloring grouts.

If you do not like the color of your grout you can correct it with cement stains – I often add black stain to my charcoal grout to blacken it. I prefer the powdered to the liquid stains.

powdered and liquid cement stains

Powdered and liquid cement stains

You can also use acrylic paint to make very colorful grouts. However, I have observed acrylic – cement mixtures do not always age well and tend to shrink and crack.

Mixing thinset and grout

I use mixtures of thinset and grout to :

  • Fill up big gaps between tiles (over 3/8” – 10 mm)

  • Pre-grout reverse method mosaics

Filling big gaps

When you have big gaps (over 3/8” ~ 10 mm) between tiles, sanded grout may not be structurally strong enough. This problem is solved by mixing grout and thinset together (or sometimes using thinset alone instead of grout). Adjust the color with cement stains, and remember that because thinset cures much slower and harder than grout :

  • you will have much more time to clean up your mosaic
  • but do not wait too long to clean it…

Pregrouting reverse method mosaics

On reverse method we build our mosaic upside down. Then we apply thinset on top of it to glue its support above it. Once the thinset has cured, we flip the mosaic. Sometimes the thinset shows between the tiles, leaving no space between them to apply the grout of the right color. To avoid this, before the application of thinset,  I “pregrout” my mosaics by applying on top of the mosaic a mix of thinset and grout on top of the tesserae.

Pregrouting a reverse method mosaic.

I apply a very liquid 50%- 50% mix of thinset /grout to the back of the mosaic. Once it is cured, I smear the thinset over it to glue the support on top of the mosaic.

I will dedicate a post to pregrouting in a near future.

On my video “Mounting Blue Carole” you can see :

  • Pregrouting (time 4s)
  • Thinset application : (time 1mn 44s)

Share this:
How to Mosaic
610 Mitchell Street, Headland, AL 36345 Map
cement, grout, mortar, pregrout, thinset
Previous Post
Mounting a mosaic with hanging rails.
Next Post
Creating mosaic models – Graphic softwares

Related Posts