This clever invention, where one pan sits inside another containing hot water, is at least 2,000 years old. Apicius, in his first century Roman Cookery Book, recommends cooking a patina, or sauce, "over a low fire or in a bain-marie", according to the translation by Barbara Flower and Elisabeth Rosen-baum. Bain-marie, which means water bath, derives from the Latin balneum mariae. And evidence of the Roman bain-marie comes from excavated cooking pots, some of them protected from the direct heat of the fire by being placed inside another larger pot that was blackened by the smoke.
In French cooking, a bain-marie is indispensable when preparing classic butter sauces such as hollandaise and beurre blanc. And large French restaurants commonly use a commercial size bain-marie that holds several small saucepans to maintain a gentle heat without spoiling the food.
So was it the well-known influence of the Auld Alliance on Scottish cooking that made a bain-marie a necessary item in the well-equipped kitchen described by Elizabeth Raper in The Receipt Book, first published in Scotland in 1756? Or was it the importance of porridge in the national diet? In practice, the lidded bain-marie or double boiler was so identified with cooking porridge that it became known as a porringer. And for making smooth porridge in the good old-fashioned way, with medium ground oatmeal instead of quick- to-cook rolled oats, a porringer is invaluable.
Moreover, porridge made in a lidded double boiler can be left to cook in a low oven overnight with no fear of its drying out. Pour hot water into the base of a double boiler. For two servings, measure 570ml/1 pint of cold water into the top pan and stir in (traditionally anti-clockwise) 55-85g/2-3oz of medium oatmeal. Cover with a tight-fitting lid and place in a low oven overnight. In the morning, stir the porridge and pour it into breakfast bowls - and don't forget to fill the porridge pan with cold water. Serve the porridge with salt or honey or sugar, and cream or milk. When you come to wash up the pan it shouldn't be burnt or corroded with the cooked mixture.
It was a different story with Wittgenstein's porridge saucepan, one which scarcely bears thinking about. The great philosopher made himself porridge every day but - his mind on higher matters, no doubt - never bothered to wash the pan. So he used the same unwashed, porridge-encrusted pan each time. Every day the space in the centre of the pan grew smaller and more horrible - it's enough to put you off philosophy for ever.
Present-day double boilers are manufactured in stainless steel, and in heavy gauge enamel. With care, a high-quality double boiler should last all your cooking life.
My Sixties aluminium version is still going strong and was only ousted in my affections by an improved design, though it is in fact a revival from the 19th century. This is a double boiler with a porcelain liner, which provides better insulation and disperses the heat evenly from the simmering water below.
I use this gentle cooking method for making delicate sauces, melting chocolate, for making fruit curds and cheeses, and most frequently for producing proper egg-based custard. This has to be one of our great national dishes because even the French recognise it as creme anglaise.
English egg-custard sauce has a pouring consistency and is not a thick mixture. To make it: heat 425ml/15fl oz creamy milk (for a rich sauce use half milk/half cream) in a double boiler until it is almost boiling. Whisk four egg yolks with 30g/1oz vanilla-flavoured sugar and gradually whisk in the hot milk. Return to the pan and cook, stirring all the time with a wooden spoon or a balloon whisk - the mixture must not boil or it will become granular and be spoiled - for five to eight minutes or until the custard thickens sufficiently to coat the back of a metal spoon. Immediately remove the pan from the base of the double boiler and pour the custard into a jug or bowl.
For a more intense vanilla flavour, heat the milk with a split vanilla pod and remove it before adding the milk to the eggs. The sauce can be given a subtle flavour by stirring in one to two tablespoons of a fruit liqueur such as Grand Marnier or Cointreau.
Use English custard as a hot sauce with baked fruit, and steamed or boiled puddings. When cold, serve the sauce with fruit tarts, sliced chocolate parfait, or in a trifle. The cold custard also makes a pleasing custard cream: soften three leaves of gelatine (or one tablespoon of powdered gelatine) in two tablespoons of sweet white wine or sherry, then warm until dissolved. Stir into the cooled custard and pour into a bowl to set.
The flavour of all fruit curds is most delectable when freshly made. If necessary, improvise a double boiler by placing a heat-proof bowl over a pan of simmering water.
Makes 340ml/12fl oz curd
115g/4oz caster sugar
2 fresh new-laid eggs, size 2
115g/4oz unsalted butter
Wash and dry the lemons. Finely grate the zest into the top half or liner (which should ideally be ceramic) of a double boiler of simmering water. Add the strained juice of the lemons, sugar and eggs.
Place over moderate heat and cook, stirring all the time, with a wooden spoon or a willow whisk for 10-15 minutes until the mixture is thick enough to coat the back of a spoon. Remove from the heat and stand the liner, or bowl, of curd in cold water. Gradually beat in the butter, adding it in small pieces. Spoon the curd into a small pot or jar. Cover, label and store in the refrigerator for up to three weeks.
To make Lemon Curd Ice-cream, fold enough freshly made lemon curd to give a good flavour into a bowl of whipped cream and freeze.
For Lime Curd: instead of two lemons, use three limes
St Clement's Curd: replace one lemon with one sweet orange
Raspberry Curd: replace two lemons with 150ml/14 pint pureed raspberries
Passion fruit Curd: replace two lemons with the flesh and pips of six passion fruit.