The original baroque polychromy, preserved in a very good state, was found on the surface of the portal of Piwna 1. It has been shown that Gotland sandstone, from which the portal is made, has low durability due to its mineralogical composition and microtexture. Specially, it is very susceptible to the action of water and temperature fluctuations. The chemical analysis and examination of the portal indicate that the polychromy had protected the stone against the aggressive influence of the environment. A fire at the portal in 1945, followed by inappropriate restoration, initiated many different processes of decay. The baroque polychromy on the portal, besides its high aesthetic and historic value, provides evidence that the paint layer on a stone surface can protect it from environmental attack. -- AATA