Dense clay can be used for sealing of boreholes and isolation of waste containers. Expansion of the clay to fill the space between containers and confining rock takes place by uptake of water of the clay, which thereby provides effective isolation. The issue is to prepare clay inserts with properly selected water content. The paper describes preparation of clay seals by mixing air-dry clay powder with nanoparticles of water droplets coated with very thin shells of a hydrophobic silicious substance ("dry water"). It behaves as dry powder and is easily mixed with dry clay. On compaction to the desired density the shells break into microscopic fragments while water becomes homogeneously distributed in the mass. Laboratory tests verify that the properties of clay prepared in this way are the same, or better, than of commonly saturated clay. © 2017 19th ICSMGE Secretariat. All rights reserved.