Cooking it to the point it will set
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Batch size is a constraint, not a preference
This is the instruction most often ignored and it is not about convenience. A large batch takes longer to reach concentration, and the extra time at heat degrades the pectin you are relying on to set it. The published guidance puts a ceiling on batch size for that reason rather than out of caution.
Making too large a batch at one time is a published cause of a soft set, and cooking too large a quantity at once is a published cause of a darkened preserve. The same variable produces two different failures, which is the strongest argument in this section for a wide pan and a shallow depth.
The steps
The sequence is short and two steps in it are where batches are lost: the point at which a boil counts as a full rolling boil, and the point at which the pan comes off the heat.
- Combine the fruit or juice, the sugar and the acid in the quantities the recipe gives for that fruit.
- Bring to a full rolling boil over high heat, stirring.
- Wipe any undissolved sugar off the inside wall of the pan before it has a chance to fall back in.
- Hold the boil and begin testing. Which test to use is on the next page.
- Stop at the reading, not at a time.
Why sugar on the wall matters
Crystals on the side of the pan are not cosmetic. They seed the whole batch, and a batch that grains is not recoverable by cooking it further. Wash the sides down with a wet brush early rather than watching for crystals, because by the time they are visible enough to worry about they have usually already seeded what is below them.
Undissolved sugar sticking to the sides of the pot is a named cause of crystals in the finished product. It dries on the hot wall, drops back in late, and gives the syrup something to crystallize around.