River rocks make beautiful veneers on walls or fireplaces, but their smooth surfaces make it difficult to anchor them firmly to a substrate, especially if the mortar joints are deeply raked. That’s why I “prime” the backside of each rock to improve adhesion.
First I prepare a very thin (milklike) slurry of water and plastic cement. Plastic cement is sold at any well-stocked building-material supply house that carries masonry products. It’s not as strong as portland cement, but it has a plasticizer added to it that makes it stickier. Plastic cement is also a little more expensive than common cement.
After soaking the stones in plain water, I place them face up in a shallow pan containing the slurry mixture — about 3 in. deep or so. I want only the backside of the stones to touch the slurry. After a five-minute soak, the stones are ready to be set with ordinary mortar. Using this method, I’ve never had a stone come loose.
Bill Hart, Templeton, CA
