I wanted to serve iced tea at a party recently, but was concerned that if I used ice to keep it cool, it would become watery.  A friend suggested making the ice cubes out of the iced tea.  For some reason the ice cubes were all crumbly and grainy.  What was going on?

--Patricia

In essence, you were making a granita by freezing the iced tea.  A granita is a frozen mixture of water, sugar and flavoring, usually coffee, wine or fruit juice.  As a mixture of water and sugar is cooled below freezing, crystals of relatively pure ice are formed.  The ice crystals remove water from the solution, making it more concentrated.  The more concentrated it becomes, the lower the freezing point of the remaining liquid.

If the initial liquid is not too sweet, large ice crystals will form, with small amounts of sugar syrup between them.  If the sugar concentration is higher from the start, then the ice crystals will be smaller and the amount of sugar syrup in the final result will be greater, at normal freezer temperatures.

The same thing was happening when you froze the iced tea.  The sugar contained in it was enough to keep the water from forming one large ice crystal, instead making ice grains interspersed with sugar syrup.  Because the concentration of sugar was fairly low, the ice still froze into a cubes.  At higher concentrations, it would have made slush, like a real granita.