Personal finance: Barman - there's a frog in this pint

Thirteen frog mugs will be on offer at Sotheby's next Thursday, when two big private collections, one of Wedgwood, the other of Sunderland lustreware, will provide market landmarks for collectors. John Windsor reports on the biggest and most comprehensive private collections of their kind to hit the auction rooms for years.

Sunderland frog mugs, together with Toby jugs and puzzle jugs (which have several spouts) have been favourites of the tavern for more than two centuries. Drinkers who were a little "canned" ("can" was the Georgian word for a mug) would be surreptitiously supplied with a mug of ale concealing a realistic ceramic frog climbing from the bottom, as if to jump into the drinker's mouth.

Some of the frogs were hollow, so that they burped and spat beer as the last dregs were drained. Murky 19th century beer added to the element of surprise. The novelty of the joke has declined, but not its ability to startle the befuddled - which may explain why frog mugs are so scarce: they ended up in pieces on the taproom floor. To avoid calamity, some bore mottos such as: "Tho' malt and venom/ Seem united/ Don't break my pot/ Nor be affrighted."

The Sunderland frog mugs at Sotheby's are among 436 lots of Sunderland lustreware collected by Ann Tolson, who spent a day a week in the early Eighties searching markets and antique shops for the 19th century pinky- purple jugs, plaques and mugs with transfer-printed vignettes and quaint mottos, often chosen by their buyers.

One late-19th century frog mug in the sale bears a characteristic transfer- print of the Wear Bridge and a hand-painted motto in the cartouche: "Distress me with those tears no more." The puce-brown frog inside has red eyes. A customised gift in the loving-cup tradition that misfired, perhaps? Lucky to have survived, it is estimated at pounds 150-pounds 250. Others are pounds 150-pounds 200 or pounds 220-pounds 320 for two lotted together.

It's a good opportunity to pick up the basis of a collection without traipsing around. As an investment, they will maintain their curiosity value - more so if you fancy researching and publishing. A book on frog mugs by Marjorie Davies was never published, but Stoke on Trent City Museum, which has her collection of 300 frog mugs, has published a pamphlet, Hops and Venom, based on it.

Sunderland lustreware designs have a nautical flavour, especially the jugs, which explains inscriptions naming distant coastal towns. Crews of colliers shipping coals from Newcastle would take orders for Sunderland motto jugs at whatever ports they visited. A beautifully potted specimen in the sale bears the legend "Willm & Sarah Drudge, Noak Farm, Isle of Wight". The names add interest. Are there still Drudges on the Isle of Wight? The jug is estimated pounds 800-pounds 1,200.

The markets for both Wedgwood and Sunderland lustreware have held their value over the past few years, and now the scare over fake Wedgwood Jasper and black basalt has blown over, the Americans and Japanese are bidding as strongly as ever.

The Wedgwood collection to be auctioned, some 450 pieces, comes from the United States, where it was accumulated by three generations of the Rinaldo family. Ben Rinaldo produced Hollywood's first all black cast movie, Dark Manhattan, in 1937.

It is a comprehensive but modest collection, short on high-priced items such as big Jasper ware plaques, that can fetch pounds 5,000 or more, but big on must-have collectables. There are plenty of pieces by famous Wedgwood designers, such as Daisy Makeig-Jones. Her 1920 Imperial bowl in "Dragon lustre" is estimated pounds 250-pounds 350. Most lots are estimated under pounds 500.

You could carry off pieces for pounds 150 or so unless the saleroom goes mad. That's the trouble with landmark sales. "Completists" who covet particular pieces to fill gaps in their collections could bid you up to prices that will not be sustained in future sales. Or a lack-lustre market could yield apparently bargain prices that turn out to be the new going rate. Decide what you are prepared to pay before the sale and stick to it. Biggest bargain: the sale catalogue (pounds 8 incl p&p). Keep it as a textbook.

The Rinaldo Collection of Wedgwood and the Tolson Collection of Sunderland Lustre, Thursday (10.30am and 2.30pm), Sotheby's, 34-35 New Bond Street, London W1 (0171-293 5989). 'Sunderland Pottery' by John C Baker is available from Sunderland Museum and Art Gallery (which also holds the world's biggest collection of Sunderland lustreware), Borough Road, Sunderland SR1 1PP, pounds 5.95 + pounds 2 p&p. 'Hops and Venom' is pounds 1.95 + 50p p&p from Stoke on Trent City Museum, Bethesda Street, Hanley, Stoke on Trent ST1 3DW.