Making and grouting a mosaic from ordinary flat tile is relatively straight forward. However, adding three-dimensional objects such as porcelain figurines or even simple rounded stones can complicate the grouting process. Instead of using a grout float or damp sponge to wipe away the excess grout, you may need to use a gloved hand as we often do, and you may have “excavate” some objects using brushes or dental picks if they have very complex shapes with nooks and crevices. An old toothbrush can be useful for cleaning wet grout from crevices in objects, but note that brushes of all type remove grout very aggressively, so take care that the toothbrush doesn’t gouge out any grout from where it needs to be.
If your mosaic is mostly dimensional objects, then the amount of labor required to remove excess wet grout may be excessive, and the easiest way to solve the problem is to avoid the grouting process entirely. Also, unsealed porous objects such as seashells cannot be grouted without staining them with the grout.
For these situations, it is better to press the objects into thinset mortar instead of the usual glue-the-grout method used for making indoor mosaics.
Thinset is a type of concrete, so if you spread it on slightly thicker, then a little bit will press up around the sides of the objects and serve as grout between the tesserae (“tile”). The key is to not spread so much thinset so that the object sinks in too deeply or too much thinset squeezes up and around the object.
Before you start pressing your childhood treasures and precious souvenirs into the sticky wet concrete that is thinset, you will want to practice for a few minutes with some ordinary rocks on piece of scrap. That way you can figure out how thickly to spread the thinset to avoid problems and practice keeping your hands clean while working with wet concrete. Otherwise your fingertips are likely to get wet concrete on objects that aren’t easily cleaned.
Your quick experiment on a piece of scrap should use more than one size of rock and include pressing objects close together. That way you can decide how closely you want to space your found objects in your actual mosaic and see how small objects have the potential to be buried in thinset pressed out by large objects nearby.
Keep in mind that small experiments like this do not require any extra time. Whatever time that is required to do them is almost always made up for in the time they save you on your project.