You will need enough apples to fill a large kettle. Wash the apples and remove the stems and the blossom ends and cut in quarters. Do not pare—a large amount of the natural pectin is lodged in and just under the skin. Cut crab apples in half.
Put prepared apples in a large stainless steel or enameled kettle. Add cold water to almost cover the fruit—there should be about an inch of apples out of the water, but you should be able to see the water level in the kettle. Cover and cook slowly over low heat until the apples are soft.
Mash the fruit slightly while it is still in the kettle. Suspend over a large bowl a damp jelly bag or colander lined with wet cheesecloth. The juice will drip through the bag or colander into the bowl. Pour the kettle contents, fruit and liquid, into the bag or colander and allow the juice to drip through into the bowl for at least 2 hours or overnight. Meanwhile, do something else so that you will not be tempted to squeeze the bag to hurry things along. Squeezing (or pushing through the colander) will not hurt the flavor of the jelly, but it will cloud it as minute particles of pulp will come through into the juice.
When the juice has dripped through, measure out 4 cups (leave the rest for another batch) and add to a saucepan and bring to a boil. For 4 cups of juice, you’ll need 3 cups of sugar and add the sugar while stirring the jelly. Continue stirring keeping the jelly at a boil until the candy thermometer reads 8 degrees above the boiling point of water - about 220°F - or until the jelly sheets off a spoon.
Remove from heat and skim the foam off the jelly or to prevent foam altogether, add ½ teaspoon of butter after you added the sugar. Quickly ladle the hot apple jelly into hot, sterilized jars leaving ¼-inch of headspace. Wipe the rims of the jars and add lids that have been washed and dried. Add screw bands and tighten until fingertip-tight.
Place jars on rack in boiling water bath canner and make sure they are completely covered with water (1-2 inches above the jars). Cover with lid and bring to a boil. Process ½ pint and 8 0z jelly jars for 10 minutes. Remove canner lid waiting 5 minutes, then remove the jars to a towel on the counter, and cool for 12 to 24 hours. Test sealing of jars by pressing lightly in center of lid and store jars that have sealed. Any jars that don’t seal may be refrigerated and used.