We might have foreseen that the movement to radicalise the art and museum world would in time come back to bite its own children. It has happened more quickly than we thought, as witness seriously red faces at the National Portrait Gallery (NPG) last week.

Among the paintings on display at the NPG was one by French society artist James Tissot of Edward Fox-White, a well-known British 19th century art dealer who opened his first gallery in Glasgow in 1854. Last year Donald Gajadhar, a descendant of Fox-White’s and manager of the art appraisal business founded by him, noticed a statement in the gallery’s notes next to the picture of his great-great-grandfather. This stated baldly that Fox-White’s father-in-law Moses Gomes Silva, a Jamaica planter, had set him up in business from the something under £400 compensation that he had received following the abolition of slavery.

The gallery’s response was a combination of self-justification and PR-speak

Mr Gajadhar raised the issue with the NPG, asking what evidence it had of this origin of the Fox-White fortune. After some vacillation, the NPG admitted that there was actually none, and agreed to remove the statement in due course.

All well? Not entirely: this episode is still disquieting. For one thing, it raises issues about how far the post-BLM obsession with slavery connections is being taken. There are actually serious questions about whether it is necessarily relevant to one’s understanding of a person that they happened to have made money from a (then entirely lawful) slave-holding concern rather than, say, a commodity dealing company – but let that pass. The problem here is that the connection is much more tenuous. There was no suggestion that Fox-White himself had any interest in slavery: the only hard fact was that he happened to marry a girl whose father had owned slaves and on abolition in 1836 had, in common with many others, received compensation.

Here we are getting close to an almost religious idea that slavery is a kind of miasma which willy-nilly affects anyone at all with any connection, however remote, to a person or business connected with it. Of course people, including museum staff, have every right to hold such a view. But whether such a connection can be said to be relevant to the understanding of a remotely-affected person or of professional relevance to an art cataloguer is, to say the least, open to severe doubt.

Secondly, this episode says a good deal about the quality of the cataloguing work at the NPG. What happened here, it appears, is that whoever was tasked with preparing the notes about Fox-White found out that his first wife had been the daughter of a Jamaican slave-owner who had received compensation. The researcher then simply assumed from this that that compensation money must have gone to set up the son-in-law in business.

This is not the kind of work we can expect from a prestigious institution. By all means state, if you must, that the father-in-law’s compensation might possibly have gone to Fox-White even though there is no evidence that it actually did and that it might perfectly well, for all we know, have been spent on something completely different. That is what the evidence actually shows. What is unacceptable is spicing up such inconsequential facts by making arresting suggestions that Fox-White’s business was actually built on the proceeds of slavery. That is bad history and bad cataloguing.

Unfortunately, this is not how the NPG sees things. Having been caught out making entirely unwarranted statements about a major figure in the art world by a relative who knew more about the subject than it did, the director might have been expected to make a fulsome apology and promise to take steps to stop this happening again. But this is not what happened. Instead the gallery’s response was a combination of self-justification and PR-speak. ‘Having listened to Mr Gajadhar’s concerns,’ it said, it accepted that there had been ‘insufficient direct evidence to show that compensation money received by his father-in-law benefitted White’s financial situation and business’. It had, it said, ‘thanked him for his feedback and would be very happy to continue to discuss the matter with Mr Gajadhar, if there are further areas of concern.’

This is close to not being an apology at all. What it seems to imply is that everyone knows that Fox-White probably started his business with blood-money, but that his descendants were technically right to say there was no proof and needed to be humoured. It is not surprising that Mr Gadajhar has asked for a full public retraction, and so far has heard nothing from the NPG.

By its failure to come clean on this matter, the NPG has also shot itself in the foot in another way. The Fox-White portrait, like a good number in the NPG, is not actually the gallery’s: it had been made available to it on loan. It is very much in the gallery’s interest to encourage such loans.

Now just think for a moment. Suppose you are asked for the loan of a picture of a relative. Are you more or less likely to agree if you know that the result may be that your relative’s personal reputation may be trashed on the basis of mere speculation by a curator with an ideological point to make? My guess is that the result is likely to be a good many fewer pictures available for the public to view in St Martin’s Place.