For André Breton 'the art of Frida Kahlo is a ribbon about a bomb'. A major exhibition at the Tate Modern (to 9 October, 2005) features her powerful work. Many have been gripped by the fascination of Frida Kahlo: not because it is intensely beautiful or hugely grand but because it is deeply personal, intensely vivid and raw. Red raw even with her own blood and with her own tears that fall in self-portraits where her body - maimed by a violent accident that cast its shadow across her life - lies ripped open. Her beautiful face lingers most in the memory. Her expression is proud, resigned and defiant. She is vulnerable and yet composed, face half-turned away from her viewer.

In her marriage to the muralist Diego Rivera she found the basis for an enduring connection with the Mexican soil and its ancient people. Mexican pitahaya fruit plump erotically and are pierced by needle sharp flags like the steel that shattered her spine and took away her womb.

There is duality in her attempt to square her European father with her campesina mother: stiff colonial dresses are juxtaposed with the peasant costume that was her trademark. Duality in her bisexuality too, and in My Dress Hangs There, 1933 which illustrates her antithesis to American 'Fordism' with its concrete factories and obsession with sport and hygiene. Her sense that she sprang from the Mexican soil and that she would return to it gave her hope of a kind of reincarnation. This powerful spirituality, which fused east and west and placed both Gandhi and Hitler in her pantheon, provided an enduring message of hope, carved out of Kahlo's despair and desperate adversity.

The cosmology of the Love-Embrace of the Universe (1949), in which she cradles Diego as naked adult foetus while she too is cradled by the Mexican earth goddess who in turn is wrapped by a female sun and moon, seemed a most poignant encapsulation of her spiritual journey.

As the light faded that afternoon I swam great circles in the men's pond on Hampstead Heath. Trees hid the broad pond from London and the water was sweet with an earthy murkiness.

'A beautiful life is to be lived in the shadows, but with peace, order and tranquility' - these words by the painter Gwen John (1876-1939) were quoted at an exhibition about her work and that of her brother Augustus which I saw for a second time this weekend. The show was at the National Museum & Gallery in Cardiff. I was there after an idyllic family wedding at the St Pierre Hotel in Chepstow.

Gwen John's words fit both her work and with my experience of a quietly happy and ordered weekend in Wales. She believed profoundly that her art was integral to her faith, and in her last years produced some remarkable understated portraits inspired by a devotional image of Mère Poussepin, the 17th century founder of an order of nuns near her home in Meudon.

These small-scale, chalky portraits have an austere beauty. John's subjects gaze serenely out of narrowly proportioned canvases in muted greys and browns. After an affair with Rodin, the painter lived out her last years in ordered isolation in a pair of little wooden houses surrounded by green. I heard four young harp students give a recital in the gallery. Mussorgsky's Great Gate of Kiev sounded wonderfully dynamic when performed by a harp quartet, overlooked by a portrait by William Parry of his father John, 'The Blind Harper of Ruabon', (d 1782) sometimes called the father of modern harpists.

I was delighted by the empty, bright galleries with their collection of remarkable Monets, Renoirs, Cezannes and Sisleys given by the Davies Sisters of Gregynog. Paintings gave glimpses of long dead sitters' lives: the newborn, a family group by Gainsborough, August John's ferocious children, the middle-aged in their prime and Gwen John's serene elderly nuns. I watched the same life stories being told out in Cardiff's shopping streets. In the warm spring sunshine all seemed ordered and peaceful. Even the traffic moved with stately good manners.

Calm good organisation and quiet consideration seemed to mark the wedding, the official start of another couple's life together. It began with a service in the tiny church of St Peter, St Pierre, next to the hotel.

The manor of St Pierre was founded by the Normans and once briefly sheltered the crown jewels. In church the wedding party sat close-by two remarkable 13th century carved tombstones, one of which is believed to commemorate Benet, the church's first known priest. A staff sprouts leaves and is surrounded by wildlife celebrating renewal. Like generations of locals before me I touched the carving's hand that he might bring luck both to me and to the newly married couple.



Gwen John at the National Museum of Wales

Tate Gallery article about Gwen John

Just 16 paintings in this show, but what an impact they made. I expected the cherry ripe sensuousness of 'Boy with a basket of fruit' (1593-4) but these six dimly-lit rooms focus on Caravaggio's last four years before his death in exile aged just 39, in 1610. Comparison between two versions of the 'Supper at Emmaus', just 5 years apart, showed the transformation from richness and colour to a darker, more humane vision. Caravaggio's stunning chiaroscuro and drama that inspires film-makers like Jarman and Scorsese. In 'The flagellation' of 1607, the tension between Christ's soft submission and the hatred of his scourger is painful to see.

'Saint John the Baptist' of around 1608 had a knowing, sad eroticism that shifted the focus between the saint and Caravaggio's street-boy model. In his last days Caravaggio was a hunted man. He bought his way out of prison and used membership of the Order of St John in Valetta to buy the Pope's pardon.

Smooth perfectionism gives way to looser brushwork and less formality. The painter sometimes depicts himself as a hunted creature or as a seeker of absolution, or even (perhaps) as the dismembered head in 'David with the head of Goliath' (1606-10). After this period of intense creativity, Caravaggio died of fever, chasing after the ship that carried away many of his last works. The power and intensity of these last paintings, several of them on a huge scale that would show poorly here, stunned me. Attempts at analysis seemed superfluous. Nothing to do but drink them in.