I’ve noticed a chill in the air recently and it is most definitely starting to look a bit more autumnal round here. The cooling temperature brings with it a reminder that I need to start being careful where I leave my mosaics to dry if I want to avoid a common problem - efflorescence.

So what is that exactly? Well, it’s a very annoying white ‘bloom’ that appears over the surface of adhesive or grout as it dries. This means that areas of exposed adhesive that you think will dry black, actually turn out to be a patchy greyish/white, completely changing the look of the piece.

Here's an example of a section of mosaic that has been affected by efflorescence.

So today I thought it might be helpful to dig a little deeper into the causes of this and some potential solutions.

And just to clarify, I’m going to focus on cement based tile adhesive, also called thinset, as that is what I usually use in my mosaics. The problem can affect other cement based products as well though.

I also want to say that what I’m sharing here is my understanding of the issue - I’ve done a bit of reading on the topic but I am not an expert!

So what is going on?

To put things simply, the white bloom is caused by salts that have been deposited on the surface. 

Water soluble salts are present in tile adhesive. When you mix it up with water, the salts dissolve and get carried to the surface. As the water evaporates, the salts are left behind.

So how do you prevent it from happening? Well,  I’ve come across a couple of things that can help.

1. Take the water out of the equation. 

If you don't have water, the salts won’t dissolve and get carried around.  You can make up your adhesive with an admix instead of water. These are latex based products that are specially designed to improve the performance of the adhesive.

Always check the product instructions to make sure that is suitable though. Brands that produce tile adhesive usually have their own brand of admix and it is best to stay within brand.  For example, Mapei Isolastic (an admix) can be mixed with Mapei Kerabond (tile adhesive).

2. Provide a warm drying environment.

Thinset cures through chemical reactions. Cold temperatures slow this process down, giving more time for the salts to travel to the surface. So if your adhesive has somewhere warm to dry/set,  there is a much lower risk of efflorescence occurring.  

What can you do if your mosaic has efflorescence?

To be honest, your options are limited and will depend on how much it bothers you and how much time you want to spend on fixing it!

The salts do sometimes scrub off with a stiff brush, although depending on how delicate your mosaic materials are, this might not be a useful solution. An alternative is to paint over it. But really, I find this a bit painstaking and really can’t be bothered.

The best option is to be aware of the issue and take steps to prevent it happening in the first place.

So as we head towards cooler weather I have to start taking my own advice and make sure I stop leaving mosaics in the conservatory to dry. The light in there is good for working but it can get pretty chilly, and I want my black adhesive to stay black!

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