Spherification.

Sodium alginate, extracted from brown seaweed, dissolves in water as long chains carrying negative charges. Calcium ions, with two positive charges, bridge neighbouring chains — the "egg-box" structure — and the solution gels instantly where calcium reaches it (X.9). Spherification exploits this at a surface. In direct spherification, about 0.5 % alginate is blended into the flavoured liquid, which is dropped into a bath of about 0.5–1 % calcium chloride or lactate; calcium diffuses inwards and a skin forms. Because the calcium keeps diffusing, the skin keeps thickening, even after rinsing, so direct spheres — "caviar" from a syringe, small spheres — are made at the last minute.

In reverse spherification the liquid itself contains calcium (from a dairy product, or added calcium lactate gluconate) and is dropped into an alginate bath; the skin forms outside the drop, and once the sphere is lifted and rinsed in water there is no more alginate to gel, so it stops thickening — reverse spheres keep, and can be large, like elBulli's "liquid olives" (2005). Both are limited by chemistry: alginate does not dissolve or gel well below about pH 3.6, so acidic liquids need a little sodium citrate; liquids rich in calcium gel the alginate before it reaches the bath, so they are made reverse; and the skin's growth follows diffusion — thickness grows with the square root of time (III.8). The workshop forms a sphere.

Below: the thickness of the gel skin on a sphere of the diameter you choose, after the minutes you choose in the bath and the minutes it waits afterwards, by direct or reverse spherification (diffusion law, thickness = 2√(D t), with a teaching diffusivity for calcium in alginate gel; direct spheres keep gelling at half speed after rinsing, reverse spheres stop).