Mandarin marmalade

My downstairs neighbor Tony is normally quite friendly. When I run into him in the street, he says hello and sometimes stops for a chat.
On Saturday, though, he backed away from me slowly as if I were a madwoman, muttering "no thanks, no thanks" without meeting my eye. It was my fault for asking the seemingly innocent question that no-one who lives in Nice wants to hear during the month of March.
"Would you like some citrus fruit?"
Local lemons, oranges, grapefruits and kumquats, untouched by chemicals and with glossy green leaves, sell for €3.50 a kilo at the market. Yet just about anyone who has been in Nice for any length of time knows someone with at least one productive citrus tree that produces a glut of fruit at this time of year. Just when I start to long for sweet French strawberries (which have, rather bizarrely, already made their first appearance at the market), I find myself cooking up enough bitter-orange marmalade to supply the whole reluctant neighborhood.
A well-meaning friend with a lovingly-tended garden filled my shopping cart to the brim with mandarins and bitter oranges this weekend. Mandarins have a mysterious, almost exotic scent that I would love to bottle and wear as a perfume. But there is a good reason why they have fallen out of favor over the years: they are absolutely stuffed with pips. Try to juice a mandarin with an electric citrus juicer and these will fly all over the kitchen, I've discovered the hard way (I now squash them with my hands directly over a sieve).
With heaps of mandarins in my kitchen, there was no excuse not to make the most labor-intensive jam in my repertory (I've decided to ignore the bitter oranges for the time being). Thanks to Philippe's help with the slicing and de-pipping it wasn't as painful as I had expected, even if it was a little disheartening to see 2.5 kilos of mandarins become a mere 7 jars of marmalade.
Still, when I've had enough of strawberries I know I will be glad to have given those mandarins a home.

Mandarin marmalade
(Sorry, it's hard to predict the number of jars! Allow about a dozen, just in case.)

An important trick when making marmalade is to save the pips as you slice the fruit. Place them in a small bowl, covered with water, overnight. The next day, drain the pips and add the water to the jam as it cooks: it's full of pectin. Unfortunately, my pips got thrown out by accident this time, which meant that the jam had to be reduced more than usual before it set.

2.5 kg mandarins (about 5 1/2 lbs)
1 lemon
2 kg sugar (about 4 1/2 lbs)

In one or two large bowls, soak the whole mandarins in cold water overnight.

The next day, drain and discard this water. Cut the mandarins and lemon into thin slices, removing the pips. Set these aside in a small bowl, covering them with water. Place the mandarin slices in one or two large bowls, with the sugar and just enough water to cover the fruit. Set aside overnight once again in a cool place.

The next day, dump the fruit in its syrup and the pip water into a very large saucepan or copper jam basin. Bring to a boil, then lower the heat to a steady bubble and cook until the juices set when a small amount is dripped onto a plate. This can take anywhere between 1-2 hours.

Pour into sterilized jam jars. I sterilize my jars by washing them well, then placing the wet jars on a tray in the oven at 180 C (375 F) for at least 20 mins.