Sea glass is heavy, and to make a mosaic with it, you must have an especially strong medium to attach the glass. Gluing sea glass on paper or cardboard will not work; the finished product will fall apart. For a sturdy sea glass mosaic, start with a layer of plaster of Paris or other modelling compound, and craft a sea glass mosaic that you can display with pride.
Choose a smooth-sided mould such as an old cake pan. Smear the inside with vaseline or other release agent.
Arrange your pieces of sea glass on a piece of paper or cardboard that is the same size and shape as your mould. Plan out ahead of time exactly where you want each piece of sea glass in the mosaic. Mosaics can be abstract, beautiful designs. You may also choose to "draw a picture" with the different colours of sea glass. When you are satisfied with your design, move on to the next step.
Mix plaster of Paris with water in a large bowl with a wooden spoon. Follow directions on the plaster of Paris box regarding how much water to use. Spoon the wet modelling compound into the greasy pan and smooth the top with the spoon.
Work quickly. Transfer each piece of sea glass from the cardboard to the same spot in the wet plaster. Gently press the sea glass in the mix. Finish your design.
Allow to dry overnight in a safe, dry place. The mosaic should not be jostled in any way.
The next day, turn the mould upside down and pop the mosaic out. Do this carefully. You don't want the mosaic to break when it lands on the table. Try popping it out over an old pillow to be safe.
Draw a picture with different colours of sea glass. For example, green for the stem of a flower, white and yellow for the flower, and blue for the background.
Have your design ready before you mix the plaster of Paris. You don't want the compound to dry to stick while you are figuring out where you want the pieces of sea glass.