Art is a fundamentally childish activity: painters dream up images and sculptors play with stuff. It was while playing with an inflatable ball with her young daughter in the early 1960s that Maria Bartuszova had the idea of filling balloons with liquid plaster instead of air. The inspiration fed her muse for 30 years, seeding the mixed crop of biomorphic forms currently filling five rooms at Tate Modern.

Trained in ceramics at Prague Academy of Arts under communism, Bartuszova turned to plaster after moving with her sculptor husband Juraj Bartusz to the industrial city of Kosice, now in Slovakia, in 1963. Plaster was cheap and plentiful: a 1987 photo in this exhibition, the first to bring her work to a British audience, shows her with stacks of sacks of it in the studio. The natural forms she initially cast from it – raindrops, wheat grains, eggs, germinating buds and dividing cells – feel pregnant with life. She called the process ‘gravistimulation’, manipulating the gypsum as it set, sometimes underwater to counter the effects of gravity. Malleable, squidgy, dimpled by the sculptor’s hand, suspended like caciocavallo cheeses, bound with cord like Nobuyoshi Araki nudes or twisted into intestinal coils like Sarah Lucas’s ‘Nuds’ made of stuffed tights, the forms are not just palpable, they’re palpatable – they tempt the viewer to cop a quick feel. To Bartuszova, an enlarged grain of wheat is as inviting as Mae West’s lips in a Dali sofa. There are other echoes of the human body: incompletely inflated balloons terminate in nipples and wrinkly knots recall other parts of the anatomy. Some forms have unforeseen associations: to a contemporary eye, the models for her two-part sculpture, ‘Metamorphosis’ (1982), for Kosice crematorium look like oozing cheeseburgers.

In the 1980s, now divorced, Bartuszova developed a negative casting method she called ‘pneumatic’; rather than filling balloons, she inflated them before coating them with plaster, stripping the rubber to leave a fragile shell. It put the void into ovoid. Clustered together and laced with loose strands of string, the dozens of empty shells in ‘Untitled’ (1985) take on the appearance of an abandoned wasps’ nest, inhabited by shadows – an aching embodiment of empty-nest syndrome. Rodin was an inspiration, and ‘Melting Snow’ (1985), a large-scale plaster relief with embedded tree branch, is reminiscent of his assemblages; this show was originally planned to coincide with Tate Modern’s exhibition last year of his plasters. But unlike Rodin, for whom plaster was a means to an end, for Bartuszova it was an end in itself. Her few pieces cast in bronze or aluminium don’t work. Her casting method could catch a raindrop in mid-splat, but a bronze raindrop is a contradiction in terms.

Beside Bartuszova’s ‘arte povera’, the textile art of Olga de Amaral looks like ‘arte ricca’. The vision of this veteran fibre artist, born in Bogota in 1932, is steeped in the gold of the ancient Andes and the rich interiors of the Spanish colonial baroque churches she attended as a child. But the aura of luxury is superficial. As with Bartuszova, the allure of Amaral’s work lies in the tangible quality of its making.

One generalises about gender at one’s peril, but here goes: while male artists keep their focus on results, women artists give themselves over to the process. For Amaral the process is weaving, though not as we know it. In 1952, as a 20-year-old architecture student, she left her native Bogota, speaking no English, to study textile design at Cranbrook Academy of Art near Detroit. They gave her a loom and she was hooked. For seven decades weaving has formed the basis of her practice, but only the basis: walking around her mini-retrospective at Lisson Gallery you might not think her hangings were woven at all until you go round the back. ‘What I’m trying to do,’ she says, ‘is make the weave disappear.’ Coated with acrylic paste and precious metals – gold, palladium and silver leaf – their coruscating surfaces gleam and glimmer like glass tesserae in Byzantine mosaics.

Where Bartuszova drew on the microcosmic, Amaral draws on the cosmic. ‘Cesta lunar 50B’ (1991/2017) evokes the rays of the moon; ‘Strata XV’ (2009) the folding layers of volcanic lava. Pulled through warps of varicoloured threads, her wefts of coated fabric strips change hue as you move around them, the colours cascading in eddies, currents and whorls. Sometimes the alchemy is real, as in the tarnished silver surface of ‘Imagen perdida 6’ (1992); sometimes it’s illusory. Whichever, the effect is magical.

‘Everything is accidental to me,’ says Amaral. ‘An accident becomes a work.’ She achieves harmony without symmetry. Like Bartuszova’s balloon art, the whole shebang hangs on something as basic as a knot. ‘The knot to me is the base of it all,’ she explains in a film downstairs. ‘It’s a whole conversation with the knot.’