The Negative Space IV (Part I) - a Positive Negative

By way of a gentle re-introduction to the Negative Space series (click on The 'Negative' Space under Series/Themes), here are some negatives of a different kind, from the AA School of Architecture website
AA School of Architecture
Clay hand models by Lucas Williams.

I came across these during my initial searching into the 'negative' space and am inspired by the forms/shapes the negative space creates. Beautiful and fascinating forms, 'making visible the invisible', or at least what is otherwise usually considered invisible; physically defining what is inside the 'concrete' form, making tangible what is inside the material substance. I also love the immediacy of these shapes, the spontaneity of form:   

My hopes/expectations were high for the first approach, i.e. using polyurethane foam to fill the negative space, as described in previous 'Negative Space' posts. 
Disappointingly the initial resulting forms/shapes were, unlike these clay ones above, in no way inspiring, neither beautiful nor fascinating. The rather ebullient character of the polyurethane foam is - or has been to date - too forceful to work harmoniously in juxtaposition with the 'positive' clay form. Colour is also an issue, as is the feel, weight and sound of it once cured.
The next step was the decision to see whether the white clay would be a better match for the black earthiness of the 'positive' form. 
Using which building technique? ...