The enclosed helium atoms escape through small pores in the latex which are larger than the helium atoms. However, some types of balloons are labelled "helium-grade". These balloons are often thicker and have less porosity. Balloons filled with air usually hold their size and shape much longer, sometimes for up to a week. However, a rubber balloon eventually loses gas to the outside. The process by which a substance or solute migrates from a region of high concentration, through a barrier or membrane, to a region of lower concentration is called diffusion. The inside of balloons can be treated with a special gel (for instance, the polymer solution sold under the "Hi Float" brand) which coats the inside of the balloon to reduce the helium leakage, thus increasing float time to a week or longer. Beginning in the late 1970s, some more expensive (and longer-lasting) foil balloons made of thin, unstretchable, less permeable metallised films such as Mylar (BoPET) started being produced. These balloons have attractive shiny reflective surfaces and are often printed with color pictures and patterns for gifts and parties. The most important attributes of metallised nylon for balloons are its light weight, increasing buoyancy, and its ability to keep the helium gas from escaping for several weeks. Foil balloons have been criticized for interfering with power lines. Sculpture Balloon artists are entertainers who twist and tie inflated tubular balloons into sculptures such as animals (see balloon modelling). The balloons used for sculpture are made of extra-stretchy rubber so that they can be twisted and tied without bursting. Since the pressure required to inflate a balloon is inversely proportional to the diameter of the balloon, these tiny tubular balloons are extremely hard to inflate initially. A pump is usually used to inflate these balloons. Decorators may use helium balloons to create balloon sculptures. Usually the round shape of the balloon restricts these to simple arches or walls, but on occasion more ambitious "sculptures" have been attempted. It is also common to use balloons as table decorations for celebratory events. Balloons can sometimes be modeled to form shapes of animals. Table decorations normally appear with three or five balloons on each bouquet. Ribbon is curled and added with a weight to keep the balloons from floating away. Drops and releases A decorative use for balloons is in balloon drops. In a balloon drop, a plastic bag or net filled