Mix the sugar with the powdered agar. Whisk into the cold water.
Add the glucose syrup. Cook over moderate heat, stirring, until the batch reaches 106°C/223°F, or 78° Brix. This takes approximately 8 minutes of gentle boiling.
Remove from heat and allow to cool to 90°C/194°F. Add the fruit juice or purée and pour into a frame 12 × 12 × ½in set on oiled parchment paper.
Allow to set until firm, about 1 hour. Cut into rectangles, using the 15-mm and 30-mm strings on a guitar.
Dredge the rectangles in granulated sugar.
Store overnight on an icing screen to allow the outside to dry before further storage.