Which materials are permitted for forming test specimens?

The selection of materials for forming test specimens is an important aspect of evaluating the properties and performance of refractories. Metal or plastic is the correct choice for forming test specimens because both materials provide the necessary structural integrity and stability required during testing.

Metal is particularly advantageous due to its strength and durability, which ensures that the specimens can withstand various testing conditions without deforming or breaking. It also allows for better thermal conductivity, which is essential when assessing refractories' thermal behavior.

Plastic, on the other hand, is valued for its versatility and ease of manipulation. It can be molded into precise shapes, allowing for uniform specimens that are critical for consistent testing results. Additionally, certain plastics can simulate certain thermal and chemical environments that refractories may encounter in real-world applications.

The other materials listed are less desirable because they do not meet the necessary criteria for specimen formation. Wood can be influenced by moisture and temperature changes, leading to warping or degradation that affects the test results. Concrete, while strong, is not suitable for the precise requirements and can introduce variables that compromise the integrity of the test. Glass might be too brittle, making it unsuitable for the stresses involved in testing refractories.

Thus, the combination of metal and plastic is optimal