Grape Marmalade

From 1913:

Pick the grapes from the stems, and measure them. Allow a half pint cup of sugar to an equal quantity of grapes and then pulp the grapes. Do not throw away the skins. Put the pulp over the fire in a double boiler or a porcelain lined kettle and cook gently until it is so tender that the seeds come out when the grapes are rubbed through a colander. You must use a fine colander, which will not permit the seeds to go through. Return the seedless pulp and the juice flowing from it to the fire with the skins of the grapes, add the sugar, and cook until the mixture is very thick; take from the fire and seal in jars.