The Importance of the Right Sand in Your Plaster

Written by Andrew Morrison

failed plaster sandThe sad and ugly aftermath of using the wrong plaster sand! In a recent workshop we discovered that the sand that was ordered for the plaster was not acceptable. I blame myself for this mistake as I did not catch the problem in time and allowed the plaster to be mixed. As is always the case, a mistake can be either just that: a bummer, or it can be a learning piece. What I learned in this scenario is the importance of finding the right sand for your plaster mix.

I spoke with the sand yard directly and told them what I needed. The dispatcher seemed to understand completely and the next day, 10 yards of angular, variable size sand was delivered to our very remote building site. There was no sending it back if we didn’t like it, it was what it was and it turned out to be the wrong stuff. The issue was that it had no fines in it.

The right sand for the Natural Hydraulic Lime (NHL) plaster needs a gradation of sand from the fines to the more coarse. No silts and clays mind you, but the finer end of the gradation is important.

You can see in the picture how the plaster simply did not hold together well. The lack of finer sand made the plaster loose and almost gravelly. I slumping plaster on a straw bale walldid a stretch test on the plaster by pulling a hawk’s worth of plaster across the table with a trowel. In quality plaster, the mud will spread and stay together, creating a smooth surface. In this case, however, the plaster ripped and tore as I pulled it on the mud board. There was nothing to hold it together.

We tried for an hour or so to get the mud to stick to the wall and eventually the sad truth became all too apparent: the plaster was not good. The worst part of the story is that we had premixed 18 bags of plaster the night before and all of it was wasted! The host now has a very fancy NHL carport floor! I ordered 18 replacement bags for the host and have informed him of the correct sand he’ll need, moving forward.

When you order your sand, be sure it meets the requirements on the NHL bag. Where I live the perfect sand is called “concrete sand.” It is angular, has many different grits (including the finer sands), and is perfect for the proper adhesion of the plaster. The same has been true across the country as I teach. The term “concrete sand” is almost always understood. In this part of New Mexico, that was not true. My plan moving forward is to send future workshop hosts a sample of the sand they should be looking for so they can make sure, well in advance, that they have the right stuff.

sand shown to scale with tape measurePer Martin’s request, here’s an image of the appropriate style of sand to use (size wise). I’ve also heard from an expert that using a lime based sand can cause problems too as the free lime in the sand will take up the moisture and leave the NHL without the ability to attach itself to the sand.

8 Responses

  1. Thanks for the post Andrew,

    Good for you and us to turn your mishap into helpful advice. I think it’s plenty common for a supplier to use whatever material is locally available, be it ideal or not, and just tell you it’s what you are wanting. These days not many people are plastering, so most customers are using their aggregates to pour structural cement where it’s assumed as long as it’s hard and gray and most importantly finished, it’s A-OK. Not many people are discerning enough aggregate customers to show up at the yard prior to purchasing with the skills to identify the adequacy of the sand mixture.

    This is probably more prevalent in rural locations especially with unfavorable geology. I’m a mason building a strawbale house on the Kenai Peninsula of Alaska. The peninsula was largely deposited by sediments from the Yukon River when it was diverted south by a massive icedam such that it emptied into Cook Inlet. Our ensuing Mississippi mud is highly rounded, and because its such a long truck to escape our geological past, everyone builds with it. Bummer for a mason and plasterer who cares a lot about the strength of his mortar. I’m desperately in need of a secret local source, but then again I’m also still holding out hope of finding the unknown hot spring that would truly make Homer a Shangri-La. Barring miracles, the reality is it will be a goose-chase and cost more to import the right stuff.

    Thanks for the picture, Andrew- your so on it. It’s utility is pretty limited however. What we need is an easy test to determine proper gradation. First you need to know the ratio by weight of each specific grain size. Then I suppose to dial it in you bring your scale and weigh out a sample size. Then you proceed to sieve the sample through the largest screen down to the smallest screen and weigh each group. It would be very helpful if anyone can specify the particle size range and proportions, I would think there would be at least three mix designs for base, brown and finish coats, which in my understanding pretty much entails reducing the maximum particle size, while the smaller end of the spectrum remains the same. I’ve also wondered what size and gradation is preferable for an adhesion coat.

    Maybe a simpler tool would be a heavy duty laminated plastic sheet that has vividly clear particle sizes printed on it. You could put your sample on it for direct comparison. One side could be in stripes of descending particle size taking up the appropriate percentage of the surface of the card, and the other could be all sizes mixed together as they will come in the sample, again in the proper ratio.

    Additionally you could have a card with actual sand particles glued to it, perhaps also two-sided, as the 3-D will be easier for some to make a comparison with.

    Then again, you should also be able to employ the same jar shake test that you use to determine the makeup of clay soils, as the heavier particles would settle faster. I imagine the delineations might be a bit more intermixed, as all sand, even the smaller sizes, settles pretty darn fast, in comparison with say sand versus silt versus clay, which stays in suspension for a day or more.

  2. Hi Andrew, Thanks for the blog. Interesting the importance of the correct sand type/composition. Would you kindly post a close-up photo of the sand sample showing the particle sa bit more openly . Say a small pile of sand and some spread/opened up a bit beside each other on a white sheet of paper to show some depth of field to the particles and make up of the sand. This would be very helpful to me so I can relate visually. cheers Pete.

  3. Hi Pete. Unfortunately, I don’t have any sand with me these days. Once I get started in the workshop season again, I will try and remember to take the pictures you request. Another option is to take the information in the post to your local sand yard and ask them to show you the right stuff. This may be more helpful as each local sand yard carries slightly different stuff, even within the same rating.

  4. Plaster sand (ASTM C-897) is different gradation than “concrete sand” which is a different ASTM #. Optimum plaster sand is actually a specific range within the C-897 specs. In other wrords sand that passes ASTM 897 can be either pleasant or unpleasant to work with. We used to source excellent sand from the Santa Cruz CA area but they now only produce it in very limited quantities. Other quarries seem uninterested in a good plaster sand because 95% of all sand goes to concrete rather than plaster.

  5. Hi Ron. Keep in mind that sand for lime plaster is different than sand for cement based stucco. This is because lime plaster needs the mechanical bond provided by the different grain sizes more than the cement based stuccos do. That said, I don’t know the ASTM range for lime plaster as I have always used sharp, clean concrete sand with great success.

  6. Hello,
    Where would I be able to purchase plastering sand? I am in Buffalo, NY. I need about 2-3 yards of it for an historic restoration project. (Frank Llyod Wright’s Greycliff) I’ve been having a hard time sourcing a supplier for my area.
    Thank you,
    Geri Ann

  7. I would contact your local sand and gravel company and ask them what they have. The type of sand you use will depend on the plaster you are working with. For example, I suggest sharp, angular, well graded “concrete sand” that has grain sizes up to 3/16″ for NHL plaster. On the other hand, for cement based plasters, you would want a much finer “mason sand”.

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