The first test used a tight weave fabric stretched through a reusable frame form. (While I didn't notice it in the first pour, there is no honeycombing in the surface of the concrete where it touched the fabric.) My presumption is that the air-bubbles were able to escape through the fabric. - This also exhibited more "pillowing" at the edges; I think this was because the fabric did not stretch as tightly as the materials in the subsequent tests.
The second test changed to plastic wrap as the stretched membrane. The surface is shiny and smooth, though picked up all of the micro-wrinkles in the plastic wrap. It also has substantially more honey-combing from air trapped by the plastic. (I did not vibrate any of the forms in this series.)
The third test used vellum as the stretched material. This cast had crisper edges, and was smoother across the whole surface, but did not have the glassy sheen that the plastic provided.
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