# Grout question



## poolecw (Oct 9, 2008)

Just installed tile in bathrooms and started grouting the joints. While still wet, the grout was the color we wanted. But the following day, the grout lightened tremendously. I knew it would lighten up a shade or two, but what was supposed to be a medium brown cured out to be a very light beige. Right now, it is nowhere close to the color it is supposed to be!

Is this common? Will applying sealer make it a little darker.


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## DUDE! (May 3, 2008)

I think maybe you could get more feed back if you put this under tiling, maybe the mods will move it for you. I've only tiled once so you don't need any advise from me. good luck though


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## JazMan (Feb 17, 2007)

When grout is wet it should be much darker than the color chart. I think you may have used too much water in the cleaning process? If you did wash it too much and with too much water, much of the pigments were washed away.

Applying a 'regular penetrating' sealer will do nothing except seal the grout. You can try an enhancer sealer such as this one. AQUA MIX STONE ENHANCER. http://www.aquamix.com/products/pg_detail.asp?pdid=32973&pgid=37155

Jaz


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## Nestor_Kelebay (Jun 17, 2008)

Poole,CW:

I'd wait until the grout is fully dry, and then apply an acrylic film forming sealer to it to see what colour it is when that sealer dries. I'd apply that sealer in a small area in a relatively dry part of the bathroom floor.

If you are satisifed with that colour, then apply that sealer to all your grout to get a permanent "wet look".

The results are going to pivot on whether the sealer is actually absorbed into the grout (by capillary pressure) just like a drop of water would be, or simply forms a film over the surface of the grout. I expect it'll actually be a little of the former and a lot of the latter.

You don't need to know the rest... It just explains why grout sealer inside the grout will darken it, but a film of grout sealer on top of the grout won't darken the colour of the grout.

It's intuitive that wet grout would be darker than dry grout. After all, wet sand is darker than dry sand, wet blue jeans are darker than dry blue jeans and wet hair is darker than dry hair. In this post, I'm going to explain why stuff (like your grout and blue jeans) is darker when it's wet.

It's entirely because the refractive index of liquids (like water) are closer to that of solids (like cement and sand and cotton) than they are to that of gasses (like air).

As a result, incident light hitting a wet material bends less at every solid/liquid interface than it does at a solid/gas interface in a dry material. As a result, light travels in a straightER line inside a wet material, and so more of the indicent light travels in a straight line deeper into the wet material and so more of that light is absorbed rather than being reflected and refracted back to your eye. So you see less light coming back to your eye from a wet material.

In a dry material, the light gets bent through a larger angle at every solid/gas interface, and so it takes proportionately fewer refractions to bend that light through 180 degrees so that it gets sent back out of the material toward your eye. Consequently, you see proportionately more light coming from a dry material. And, the light you see is WHITE light. That's cuz at every solid/gas interface, the light gets refracted just like it does in a prism, with each different frequency of light getting refracted through a slightly different angle and being sent in a slightly different direction. Your eye sees that mixture of different colours (frequencies) of light as the colour "white". And, when you mix white with any other colour, you get a lighter version of that same colour. Mixing red and white gives pink, for example.

(You don't need to know this: And, it's entirely the fact that light travels in a straightER line through wet material than dry material that makes wet T-shirt contests possible. Because light is traveling in a straighter line, it behaves more like it would if the wet cotton weren't even there and it were traveling through air. This is why wet cotton is more transparent than dry cotton. You can prove this to yourself by putting a drop of water on a piece of tissue paper. The wet area will look darker than it's surroundings UNTIL you hold the tissue up to the light, when the wet area will become much lighter than it's surroundings. That's because wet materials are more transparent than dry materials cuz light travels in a straighter path through wet materials cuz of that refractive indian stuff.)

So, the change in colour of your grout is really an optical illusion. Your grout has whitened for exactly the same reason that clowds, snowbanks, waterfalls and the head on a beer are all white, even though nothing inside any of these things is white in colour. It's the fact that your grout is scattering more light now that it's dry that has made it appear lighter (er, I mean "whiter") in colour.

However, as Jazman says, it could also be that you may have washed too much pigment out of the grout due to excessive sponging down of the grout. The way to tell is to wait until the grout cures, and then put a drop of water on the grout in an inconspicuous spot in a relatively dry area. If the grout returns to it's original colour when wet, then we're just dealing with optical illusions here, not damaged grout. 

By painting a grout sealer onto your grout and allowing it to be wicked into the grout by capillary pressure and dry clear inside the grout, then you can, hopefully, give your grout a permanent "wet look". (cuz the refractive index of a clear solid plastic will be even closer to solid like grout than water is)

Here's a grout sealer I like because it's 100% acrylic, non-yellowing and it dries very hard:
http://www.glaze-n-seal.com/images/lg_grout.jpg
http://www.glaze-n-seal.com/docs/factsheets/Grout Sealer FS41.pdf

Now, in order to optimize this gameplan, we have to have as much grout sealer absorbed into the grout as possible to fill the voids between the grout particles as completely as possible with plastic. That means having the maximum capillary pressure in the grout, and that means applying the grout sealer when the grout is bone dry. That's because the formula for capillary pressure in a porous media is different than that for a drinking straw.

Warning: the following paragraph gets really ugly. Parental discretion is strongly advised.

Imagine two spheres in contact. If you were to put a drop of water at that point of contact, the water drop would form an odd shape called an "annular ring". It would be concave on both sides (cuz of the shape of the spheres) and concave around it's perimeter, too. If we call R1 the radius from the point of contact of the spheres to the bottom of the meniscus of the perimeter of the annular ring, and R2 the radius of curvature of the perimeter of the annular ring, then the capillary pressure is given by:

Pc = S ( (1/R1)^2 + (1/R2)^2)

(or that's as best I can remember it. I couldn't find a web site that gave the formula for capillary pressure of an annular ring)

Where S is the surface tension between the water and the material the spheres are made of, 1/R1^2 is the reciprocal of the first radius squared and 1/R2^2 is the reciprocal of the second radius squared.

So, you can see that as the droplet of water dries up, the annular ring becomes smaller and smaller and smaller, and both R1 and R2 become smaller, making the calculated capillary pressure higher and higher until it eventually becomes infinite once the droplet completely evaporates. (It doesn't really become infinite, it becomes "undefined" because you can't have any capillary pressure if there is no liquid to get sucked into a crevice or up a straw.)

So, according to the math, you want to apply the grout sealer when the grout is as dry as possible, and that means waiting as long as possible for the water in the grout to evaporate out of it. That will mean that the grout will have it's highest possible capillary pressure and will absorb the most grout sealer (for the best results).

And, that's the clincher. If the grout sealer only gets sucked a tiny way into the grout, and mostly forms a film over top of the grout, you're not going to get that colour change you're after. You need the grout to suck up the sealer like a sponge so that you can get light traveling as straight as possible inside the grout for as far as possible to get the most light absorbtion, and the greatest darkening of the grout's colour.

Hope this helps.


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## poolecw (Oct 9, 2008)

I bet that I over-wiped the grout. You would think that the color would be uniform thoughout the grout and that wiping away material would only expose more of the same material.. Luckily, I've only grouted two small bathrooms, I still have the kitchen, utility room, and master bath to try to "do better".

Nestor, I'm an engineer with a minor in physics. Please don't bring up those kind of memories anymore!


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## Bud Cline (Mar 12, 2006)

> Pc = S ( (1/R1)^2 + (1/R2)^2)​
> "*or that's as best I can remember it."*​


 

Hahahahahahahahahaha! Or is it: Hehehehehehehehehehe?

No, it must be Hahahahahahahahah! THAT'S THE BEST I CAN REMEMBER IT.:yes:

Copy and paste is a wonderful tool!


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## Termite (Apr 13, 2008)

Nestor, only you can turn a question about grout coloration into a physics lesson with an actual equation involved. :laughing::no:

As for the color, I've experienced the same thing. Had the wife go to the tile store for grout and she selected natural gray. Not knowing exactly what natural gray looked like I took her word for it that it looked good and grouted the project...Only to be shocked at how light it was once it dried. Could've killed her! Enhancer/sealers will darken it about one shade, but in my experience the results won't be astounding.


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## DangerMouse (Jul 17, 2008)

couldn't they use a stain to darken to near right color, then a sealer will darken a hair more?

DM


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## poppameth (Oct 2, 2008)

One of these companies, I think Aquamix, makes an epoxy grout dye that would do the trick for you. It'll dye the grout lines whatever color of dye you choose, plus it will permanently seal the grout against staining. Comes in a kit that includes an application bottle with wheel to roll the lines with.


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## DangerMouse (Jul 17, 2008)

stain and sealer in one? cool poppa, that eliminates the guesswork between coating with stain, then sealer. thanks for the post. Po)

DM likes epoxies....


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## poppameth (Oct 2, 2008)

Here's the link to it.
http://aquamix.com/products/pg_detail.asp?pdid=25042&pgid=8374


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## 26yrsinflooring (Jul 1, 2008)

Grout Physics 101 

Good answer Nestor.

Follow Poppa's link, I used some of that on a job recently, worked great.


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