Use a potato peeler or sharp paring knife to remove the zest from each piece of citrus fruit.
Chop all the zest and add to the pot.
Next, remove all the pith from each piece of whole fruit using a sharp paring knife. And then cut each section ("supreme") of fruit from between each membrane.
Add each section of fruit to the pot with the zest.
Take the leftover membrane and squeeze it over the pot to release any remaining juice.
Sprinkle 1/8 teaspoon of baking soda into the pot. This will help break down the zest and make it softer during the cooking process and release the pectin.
Place the pot on to the stove burner and add 4 cups of water to the pot. Bring the mixture to a boil and then turn down to a low simmer. You will simmer the contents for 30 minutes.
After 30 minutes, using a wooden spoon remove a piece of the zest from the pot. Allow it to cool, and then press it between your fingers. If it is soft, you are ready to move on to the next step. If it is not soft, allow it to simmer for a few more minutes until soft.
If you plan on water bath canning this marmalade, see the "Water Bath Canning Instructions" section below before you proceed.
Put a small plate into your freezer.
Transfer the contents of your pot into a large heatproof measuring cup. If you have 6 cups of the mixture, you are ready to proceed. If the mixture does not measure 6 cups, add water so that you have a total of 6 cups of mixture. Now you are ready to proceed.
Return the mixture to the pot and bring the mixture up to a boil.
Slowly add 2 cups of the sugar to the boiling mixture and continue to stir.
Once the first 2 cups of sugar have dissolved, add the next 2 cups of sugar and continue to stir. When those 2 cups have dissolved, add the remaining 2 cups of sugar and continue to stir the mixture.
Continue to boil the mixture until you can not stir down the boil. (At this point, a candy thermometer should read 220°F or 105°C.) This can take anywhere from 10 to 30 minutes.
If during the boiling time there is the development of a significant amount of foam, add the butter and stir well to incorporate. The foam should subside. If it does not subside or you decide not to use the butter, you will need to skim off the foam.
Once you can not stir down the boil, boil the mixture for 1 minute more and then turn off the heat.
Remove the small plate from the freezer and place one teaspoon of the marmalade on to the plate. Put the plate back into the freezer for a few minutes.
After a few minutes, remove the plate from the freezer. If the mixture has gelled and you can run your finger down the middle of the dollop of marmalade with each side of the marmalade not leaching back into the middle, the marmalade has gelled correctly.
If you are not canning this marmalade, transfer it to clean sterilized jars, put the lid on the jars, and refrigerate. Marmalade stored in the refrigerator will last approximately 3 months.