Combine strawberries and sugar in a flat-bottomed kettle; cook over low heat, stirring until sugar dissolves. Bring to a rolling boil. Boil, stirring and mashing strawberries, until mixture registers 220° on a candy thermometer or until mixture sheets from a cold metal spoon. Cook to a higher temperature for a firmer product, to a lower temperature for a softer product. Remove from heat, and skim off foam.
Quickly ladle jam into hot sterilized jars, leaving 1/4-inch headspace. Cover at once with metal lids, and screw bands tight. Process jam in boiling water bath 10 minutes.