A White House spokesman said that the US was considering toughening its own existing sanctions. The processes of thermal and chemical toughening can increase the tensile strength of glass. Ceramics have limited crack tip plasticity and primarily rely on different toughening mechanisms. Simmering ensures gentler treatment than boiling to prevent food from toughening and/or breaking up. The matrix crazing theory focuses on explaining the toughening effects of crazing. The crack tip plasticity is important in toughening of metals and long-chain polymers. Acrylonitrile butadiene styrene (ABS) polymer is an application of rubber toughening. When fiber is brittle, the pull-out work dominates the irreversible work contributing to toughening. Freeze-drying does not usually cause shrinkage or toughening of the material being dried. The properties of this polymer come mainly from rubber toughening. It is a way of improving mechanical properties, in a technique known as rubber toughening. In the following years, Interior Minister Charles Pasqua was noted for dramatically toughening immigration laws. The Government of South Korea is toughening visa regulations for Chinese students. There are only several examples are used to explain the toughening in polymers. Another means of toughening the bouligand structure is by shear wave filtering.