by Athena and Bill Steen - Cohabitat

Perlite (or vermiculite)/light clay
Perlite is mixed with a clay slip, either by hand or with a concrete
or mortar mixer. Enough slip needs to be added so that the perlite
binds together and that the mix has sufficient density. Like most facets
of the earthen floor making process, experimentation will be needed
to yield the proper proportions.
Pumice fines (or other lightweight volcanic stone) can most
likely be laid dry and tamped for good compaction.
The Sub-Floor
The finished earthen floor, needs to rest on a compact, level
surface. If an insulating and/or drainage layer has not been used, the
sub-floor can be nothing more than the well-compacted ground.
Otherwise, 2 to 3 inches of compacted silty/sandy soil (not pure sand)
can be applied over the insulating or drainage layer to insure a solid
base. This soil needs to have enough day that it holds together when
dry. Slightly moistening the soil will help the initial compaction. The
sub-floor can serve as a good working surface during the final
construction stages of the building. Foot traffic will help compact
the bottom layers.
Radiant floor tubing can be installed into this sub-floor layer.
Finding a Soil Mix for the Finished Floor Layer
Determining a good soil mixture, a combination of clay, sand
and fiber (chopped straw), is the most critical step of the finished
floor layer. The mixture needs to have sufficient clay to bind all the
ingredients together with good strength. Conducting a series of tests
using varying proportions of these ingredients, is a crucial step. The
strongest earthen floor is one that doesn't crack. Cracks, even when
filled or grouted remain the weakest and most vulnerable part of the
floor to damage. Creating test samples to see how they dry is crucial.
Analyzing soil for clay content
To determine whether or not soils contain enough clay to be
used for a floor, several simple tests can be done. One is to form the
moistened soil into a round egg-sized ball using just enough water to
bind it together without it sticking to the hands. Let it drop. If it breaks
easily there is most likely not enough clay content. If it resists breaking
and has a sticky adhesive quality, the clay content is probably
sufficient. The same ball can also be cut in half with a knife. A shiny
surface on the soil reveals clay while a dull surface indicates silt
Another indicator is that clayey soils resist penetration of a knife
while silty/sandy soils show little resistance. Handling and feeling
different soils and noting the differences between them will begin to
reveal a lot about them. With a little experience you will have a
natural sense about what generally needs to be added to correctly
adjust the soil.