Reticulated foamFoam is the structural material most soft puppets are built ... More is open-cell polyurethane foam that has had its cell membranes removed, leaving only the skeleton of cell walls — an interconnected web of strands with air passing freely through it.
Hold a piece up to a lamp and you can see light straight through. That is the test.
Why puppet builders want it
Three reasons, all practical.
It breathes
A puppeteer’s hand and forearm are inside that head under stage lights, sometimes for hours. Air moving through open cells is the difference between a workable afternoon and a miserable one.
It is light
Almost all of the volume is air. A reticulated foam skull weighs a fraction of the same shape cut from upholstery foamUpholstery foam is the dense polyurethane foam sold in craft..., and weight is what causes a performer’s arm to sag by minute forty — which the audience reads as the character losing interest.
It bonds and finishes well
The open structure lets contact cementContact cement is a flexible solvent-based adhesive applied ... penetrate and grip rather than sitting on a closed surface. It also sands to a genuinely soft contour instead of tearing.
How it is sold
By pore count, or ppi — pores per inch. Lower ppi is coarser and more open; higher ppi is finer and holds a crisper carved edge. Fine-cell grades like L200 are the choice for noses, brows and any sharp feature. Coarser grades are fine for bulk.
What it isn’t
Upholstery foam from a craft store is closed-cell or nearly so. It is denser, heavier, hotter, and it fights the cement. It is serviceable as a body core. It is not a head.
The hack: buy sheet, not block. Sheet foam dartsA dart is a wedge of material removed from a flat piece and ... into curves and gives you a hollow shell; block foam invites carving and gives you a heavy solid one.