Wash the oranges thoroughly. Halve them and squeeze the juice into a deep stainless steel dish. Scrape the pulp and seeds into a strainer and squeeze every drop of juice from them, then tie them up in a piece of muslin. Grate the rind and the white part on a grater, or cut it into small pieces with a very sharp knife.
Add the rind and muslin baggie to the juice, top up water and lemon juice depending on the quantity of oranges. Leave it to soak overnight, which saves boiling time. Heat the liquid and let it simmer on medium heat for an hour and a half. The rind should become very soft, slightly transparent. Remove the muslin, let it cool a bit, put on rubber gloves and squeeze the juice from it.
Weigh the pot – it should be about 28 oz (800 grams), if less, add water. Place 4 plates in the freezer, with which you will be checking the density. Add the sugar and reduce the heat. Stir and do not let it boil before the sugar completely dissolves. Increase the heat, boil the mixture and do not stir. After about 5 minutes the marmalade will start to rise and make large bubbles.
After about 10 minutes check the density (the mixture becomes ready faster in a larger pot). To check density, remove the mixture from the heat and allow the bubbles to pop. Take a plate from the freezer, put a little liquid in it and put it back in the cold for 1 minute. If the marmalade is ready, it will wrinkle slightly to the touch and will not run.
A good density is reached at 221°F (105 °C). Leave the marmalade for 10 minutes or until it begins to thicken even more. If there is foam on the surface – remove it. Put it into sterilized jars and seal. Keep the jars in a dark and cool cupboard. They last about a year.