JOHN Glassford was a practical, if not the most romantic, chap. After going to the bother of having his wife and seven children painted with himself in a large family tableaux, his poor wife died. Marrying again for the third time the following year, he merely had the face of wife three painted over that of wife two.

Hopefully wife three, Lady Margaret Mackenzie, was not too unhappy with the body shape of wife two, Anne Nesbit.

The fact that she had married the richest man in Glasgow probably helped ameliorate her concerns.

To be fair to John, this happened in the 1760s, when Photoshop was unavailable to update the family snaps. The pictorial wife-swap was only discovered by conservators at Kelvingrove Art Gallery when they X-rayed the painting of the Glassfords which had been donated to the city. You can judge for yourself how well Margaret has been included as the portrait is on display in the temporary exhibition How Glasgow Flourished in the art gallery until mid-August.

It covers the Georgian period when Glasgow blossomed into a great port, with half of Britain's tobacco passing through its warehouses, and making a few businessmen, the city's so-called "Tobacco Lords", including John Glassford, multi-millionaires. The business was huge. At its height,

47 million pounds of tobacco leaf annually was passing through Glasgow, with the bulk then being reshipped to the rest of Europe.

There is though a darker side to this period of growth. I'm not sure if it is the same portrait, but there is a painting of the Glassfords where the conservators also discovered a black servant who had been painted over, presumably when the abolition of slavery movement was making its mark in Glasgow. For although the Tobacco Lords of Glasgow were discussed in the history of Glasgow when I was at school, the fact that the tobacco trade was founded on slavery was never really referred to.

Part of the reason was that the Tobacco Lords themselves usually did not own plantations but were the middle men, who bought the tobacco from the plantation owners.

The exhibition explains that they used the "Glasgow System" in which the city's business community set up their own banks to provide credit to buy raw goods such as tobacco, sugar and cotton, and give credit where necessary to the tobacco growers, and then to invest in the factories around Glasgow making the clothes and metal tools to fill the tobacco boats to sell to the American settlers.

The exhibition curators have cleverly included a quotation from free marketeer Adam Smith who wrote: "It is not by augmenting the capital of the country, but by rendering a greater part of that capital active and productive than would otherwise be so, that the most judicious operations of banking can increase the industry of the country." In other words bankers, stop paying yourselves bonuses and start backing local businesses.

There's also a point for the independence debaters to mull over - until the 1707 Act of Union only English ships could trade with the colonies. After the Act the Glasgow merchants got stuck in, and with Glasgow being the first port of call in Britain from America, trade boomed. Not only was it a far shorter route than going via the English Channel, it was also safer as England always seemed to be at war with someone in Europe making the Channel risky at times.

Joe Fisher, the late librarian at the Mitchell who was an expert on all things Glasgow, once wrote that the Glasgow merchants were wily enough to employ someone known as a "supercargo" on board their ships. The supercargo was not the captain but the person in charge of the goods. He had the authority from the owners in Glasgow to sell the goods as quickly as possible, and to buy as much tobacco as he could, also in quick time, so that the Glasgow ships were turning around from America far quicker than their English counterparts. Three hundred years ago, the Glasgow bosses knew the value of delegation.

The Kelvingrove exhibition doesn't duck the slavery question which it might have done in the past. There are local artefacts which include carvings of slavery. Records show 70 African slaves were living in Scotland between 1756 and 1838.

To be fair, Glasgow also had one of the most active anti-slavery societies in Britain, and even the Scottish Court of Session ruled in 1766 to allow a slave to have his freedom from a Glasgow businessman, stating: "The purchaser of a negro in the British colonies has no right to the perpetual service of such negro while in this country."

As for home-grown businesses, life wasn't all portraits and hunting, if you were a mere worker. The water supply was so bad, taken from the Clyde or the Kelvin or hand pumped from wells, that people drank weak beer because there was less chance of catching a disease, or at least that was their excuse. Radicals who campaigned for workers' rights were even executed, as happened in the 1820 Radical Rising.

Weaving went from a cottage industry to a huge manufacturing industry that covered the green fields of an ever expanding Glasgow with the largest dyeing factories in the world.

The exhibition explains that as the textile industry flourished in Glasgow, one owner employed only Gaelic speakers so they wouldn't accidentally pass on how his red dyes were made. And to think all those years I've been told by the Gaels that they were sought after because they were hard workers.