Pebble Mosaic Instructions: Reverse cast “upside-down cake” technique
Pebble mosaics are a great medium for rendering aspects of the natural world from insects to animals, birds and fish, as well as for creating more abstract designs.
Pebbles – rough or smooth, tile on edge, glass balls. All sizes work but 1/2'”-1” are best
Screws – to keep forms in place
Sealer – industrial acrylic patio sealer for the wet look
Wood base – plywood or chipboard (min ½”)
Form – contains mosaic for casting, (plastic rings, wood, choroplast, bent tin) min 2” deep
Expanding grout – comes in a 50 lb. bag ($10 at building supply) –not tilesetter’s grout
(Readi-mix bagged concrete can be used to thicken slab for less cost)
Fine Sand – Must be dry, sift to remove debris
Garden Store annual trays are good for pebble display and stacking –lift in the middle
Buckets – one for mixing grout/concrete; one with water for tools and hands
Spoon and Paintbrush – (¼” soft bristle) for detail finishing
Tape – wide packing, masking or duct tape
Dust Mask &Rubber Gloves–for all concrete work
Garden Trowel – small one for mixing and scooping
Water Sprayer/Spry bottle - to wet rocks while selecting for colour
Stiff Bristle or Wire Brush
Find a good table height for your back. Children are good with standard table. Most adults need workbench height. Ideally, work on the mosaic where it doesn’t need to be moved until after it is cast and hard.
1. Sketch out a simple design on paper. Consider a border and plan rock colours, remembering to use contrast (light and dark).
2. Make mold (form). Use set screws every 4 inches for choroplast/metal forms. All forms need to be taped on the inside to keep sand , and later, grout form leaking out. Press tape into corners first and then flatten out, using fingernails for good contact.
3. Add Sand: Pour sand a little less than ¼” into the mold. Determine centrepoint and sketch design into sand with end of paintbrush. (Alternately sand can be added as you go, as much as the stones need to stand up -thereby allowing for following a design drawn onto baseboard or paper -taped to base).
4. Layout: Place pebbles into the sand following your design and push or twist each stone right down to the baseboard surface (scratchy sound) to ensure a level surface when finished. Make sure they are not buried more than half way in the sand. You only need enough sand to hold the stones in place so remove some sand if needed. More can be added later. If sand starts to come between rows of stones, work the pebbles together and use less sand. Use spoon to scoop out excess sand.
Place stones tightly together (touching) inside the form since any areas where sand is showing will become concrete. Stones can be placed flat or upright to create the line work of your design. Avoid pebbles less than ¼” high. Edges of figures are best rendered with stones upright rather than flat as the indents between roundish stones will break your line. Edge-placed stones also allow for more concrete to hold them. They also allow for rows of vivid flowing patterns.
** Remember to place the correct side of the pebble facing down. And remember any text will need to be reversed.
** Keep your work dry: use the spritzer for colour selection away from the work, since wet sand sticks to everything. At the end of the day you can lightly spritz the mosaic to check for colour
TIPS: Once in a while, stand back, stand on a chair and/or make your eyes squinty to see how the patterns and contrasts are emerging. This is how many people will experience a finished work on pavement or on a distant wall.
5. Fine Tuning: Keep sand dry. Use spoon and mini paintbrush to add or move sand to optimum depth. Maximize the surface of the pebble facing down while…For excess sand, one trick is to remove one central rock and sweep surrounding excess into hole, then scoop out. Or jiggle the pebble to let sand fall under. Or jiggle end of paintbrush in gaps. Pay special attention to outside perimeter. Look for shadowy areas. For small rocks or gravel, be careful not to bury.
6. Mixing the grout: Mix the expanding grout to a slightly fluid consistency by adding water first to the bucket and then adding the grout and stirring while pouring, until the consistency is achieved. Follow instructions on bag. Best to start with less water and add more. You want a hotcake batter consistency (fresh cow poop) -pourable but not runny. Press out any lumps. Spritz water over mosaic to stabilize sand.
7. Pouring Grout: Gently cover all the stones by pouring or scooping grout, paying attention to tall stones and sensitive areas first. A gloved hand is good for spreading out grout that ended up to thick. Get it all on quickly and start agitating with a paintbrush end as the rocking fulcrum. The grout will re-liquify as bubbles come up. 3-5 minutes.
If you want to top up mold with regular concrete, do so within 3 hours, but wait at least 15 minutes, while grout sets up. Level with flat trowel, pressing gently. Agitate as well.
If placing later onto wet concrete, roughen up surface with brush or cut marks for maximum adhesion. This may require 15 minutes of setting time for it to be stiff enough. This is a good time to put in anchor bolts for same purpose.
8. Curing: Stretch some plastic over the whole, weighed down with rocks or plywood, or use tape to seal. Let cure at least two days before examining, then re-cover and let cure another few weeks if possible. 28 days for maximum strength.
9. Expose Mosaic: Undo mold fasteners. For round molds, slip a hammer under the taped wall and lift, with form on the ground for better torque. Scrub off sand with brush. If any bleed-through of grout happened, you can now gently tap the crust out with a hammer and the point of a screwdriver. Any excess along the outside edge can be tapped off by ever-so-gently tapping inward towards the centre.
~ courtesy of Glen Andersen, Mosaic Planet
for the Arts Engage! Symposium, Halifax, NS 2010