The difference between "jam" and "preserve" is marginal, but it's generally accepted that the latter has a higher fruit content. Mark Raffan prefers batches of less than 5kg, which should, ideally, boil for about 10 minutes to reach setting point.
(For about 6 x 500g jars)
1.8kg granulated sugar
Zest and juice of 1 lemon
1tbs lemon pips (optional)
Mix the fruit and the sugar well in a large pan that will comfortably hold them. Add the lemon - and the pips, wrapped in a muslin bag, should you use them (they help setting). Bring to the boil quickly and boil hard for 10 minutes, but take care that the preserve doesn't boil over. Stir, but don't skim.
The old-fashioned way of testing whether a preserve has reached setting point is to spoon some on to a chilled plate and draw a line through it with a spoon. Mark Raffan prefers this, because raspberry is quite low in pectin and won't set solid.
Alternatively, use a sugar-boiling thermometer or probe and boil to 104-105°C.
Before pouring the preserve into jars, take off the heat and skim carefully. Seal while the preserve is hot.
Note: It is possible to up the proportion of fruit, but this means cooking times will be longer, and the flavour isn't markedly better.