waterglass for binding earthen surfaces & pigment

“Waterglass” for protection & paint

Waterglass has become my preferred binder in places where it’s needed. The chemical name is sodium or potassium silicate. It’s an inert mineral compound similar to window glass, but under heat and pressure, it’s soluble in water. I get it from a ceramic supplier for $9 a gallon. It’s clear, viscous, and pours like heavy cream. It dries into a clear, brittle substance that crushes to a fine powder, but it has significant binding power, and is used in some refractory cements, as well as numerous other industrial applications.

I’ve only discovered it in the past few years, so I’m still learning, but it has made murals possible in less protected areas where I might not have risked it before. It does interesting things with color. And it’s cheap!

Mixed at least 50:50 with water and sprayed (or brushed) onto dry mud and allowed to dry again slowly, waterglass will bind the mud to a significant depth, preventing damage from rain, hoses, and curious fingers (but not hostile ones). Brushed on, it soaks in deeper, binds more, and may darken colors.

Mixed with pigment, it produces wonderfully varied mottling: more opaque in deep areas, where the pigment settles thicker, and more transparent on raised surfaces.

A 50:50 mix will treat approximately 30 sq ft per gallon — more if you spray, less if you use a brush and really saturate the mud. More saturation provides more strength and water-resistance. If you want to stretch your supplies a bit more, you can dilute it with a little more water.

Make sure to really fill every nook and cranny, otherwise, you end up with uncemented areas which will be fragile. When applying it, especially over deeply textured mud, it’s almost as if you’re pouring it on with the brush. As you’ll see, it soaks in so fast you don’t really have time to brush it. Strange stuff.

Spray bottles are a good way to apply it, especially if you want a thinner application. Coarse rather than fine spray is less of an inhalation hazard.

• Waterglass is mildly caustic, so gloves or regular hand-washing is indicated.
• While it’s liquid, it is still silica, and bad for the lungs; if you decide to spray it, wear a mask.
• Be careful of overspray and drips, as the stuff will mar glass surfaces, and can be hard to clean off of other surfaces as well.

1. Mud should be thoroughly dry before applying waterglass.
Waterglass, clay, sand, and water make a gel before drying out completely. Perhaps because of the very binding properties that make it useful, it can take some time for that final drying to occur. If additional moisture is still moving out from deep in the wall, applying waterglass too soon may further slow drying.
So a thick mud wall may just appear to be dry. If you apply waterglass too soon, and then get rain, you may end up with soft, jelly-like patches that can even slough off completely. If you’re not sure, better to let an earthen wall dry completely – a year, if need be – before waterglassing.

2. Let the waterglassed surface dry slowly.
Say you waterglass a dry wall on a sunny day. The next day you find it covered with white powder. Some of it brushes off easily, but some sticks and gives your final color an annoying dusty finish.
I’ve heard two explanations: one is that fast drying pulls both water and waterglass out at such a rate that the mineral ends up drying on the surface where it turns into powder; the other explanation is that the waterglass displaces other salts that may be present, and those get deposited on the surface. Either way, the problem seems to be exacerbated by overly quick drying. I waterglass late in the day after the sun is low and the heat has dropped.

11 Responses to waterglass for binding earthen surfaces & pigment

  1. I am in process of rebuilding the oven. The ceiling keeps falling and leaving dirt on my pizza. Will adding waterglass directly into my first layer mix be effective in keeping my ceiling together and free from falling? If so, from your experience with water glass, is there a ratio that you could recommend for the first clay layer? Example 20% waterglass solution (50 water:50 water glass) to 80% clay/sand mixture?

    • Kiko Denzer says:

      While waterglass in the mix will help, you might also want to re-evaluate your source of clay subsoil, or your mix — lack of bonding suggests poor quality — or lack of — clay. It is also possible that your mix simply wasn’t tamped hard enough to bond clay to sand. (Many variables make many possible sources for a single problem!) On the waterglass and mix ratios, please look up the post on DIY castable refractory (now at the top of the “hot from the oven” blog). Good luck!

  2. Pingback: Building With Cob? Don't Let Your Efforts Be Washed Away! | WattseeWattsee

  3. Hello Kiko, I purchased your book and built an earth oven last year. We love it, but sadly haven’t been able to use this year. The ceiling at the entrance of the oven is starting to loosen and dirt keeps falling which makes for icky pizzas. How does one go about repairing a loose dirt ceiling? We’ve tried adding a slip, but it doesn’t seem to adhere. I’m wondering if spraying a coat of the waterglass would work, and whether it is safe to use on the inside of the oven? Any advice would be appreciated. Thank you!

    • kiko says:

      yes yes waterglass is safe to use on the inside of the oven — just re-read the precautions above. (It was reportedly used to preserve eggs, so ingestion is apparently not a worry.)

      • Alicia Arzate says:

        I just finished building my first oven and am trying to come up with a finish plaster that will give some weather protection, although I am planning on building a roof for it. Can waterglass be used as a top coat to help with rain protection? How does this compare with using lime plaster? Should I do a basic clay plaster and then just brush it on? Thanks!

        • kiko says:

          yes, it does give some weather protection — comparable, in some (but not all) ways to a lime plaster. But a roof will be best. Yes, brush it on per the directions in the article.

  4. Yachid Yada Meir Zara says:

    Does this process/finish leave the earthen wall ‘breathable’?

  5. Bruce Cumming says:

    Hello Kiko !
    Built my own oven following instructions in your book – great book,easy to read and follow.
    Thank you!
    One question : it is a little flakey inside and I get grit on the food. I am thinking of trying to use waterglass to seal the inside of the oven. What do you think/suggest ?
    Kind regards

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