Greetings . . .
"Heating the sugar and water together is satisfactory if the mixture is stirred constantly. However, the sirup should not be boiled over direct heat as it will caramelize and will not be satisfactory as feed for bees. Sometimes sirup made in this manner will crystallize in the containers. This may be prevented by adding to the prepared sirup a tablespoon of tartaric acid for each 100 pounds of sugar used."
Source: The Hive and the Honey Bee, 1963, p252
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Dave W