Which item is used for filtering substances when preparing heroin?

Cigarette filters are commonly used in the preparation of heroin for filtering substances due to their availability and capability to trap larger particles, allowing for the purification of the drug. The porous material of cigarette filters can effectively separate solid residues from the liquid solution, which is essential for obtaining a more refined product. This practice is often part of the illicit drug preparation process where cleanliness and the removal of impurities are paramount to ensure better user experience and potency.

Other options, such as steel bowls, metal strainers, and plastic bags, serve different functions in various contexts but do not possess the specific filtering qualities that cigarette filters provide for this particular purpose. Steel bowls and metal strainers may be involved in the cooking or mixing process, while plastic bags are generally used for storage or transport rather than for the specific task of filtration.