“To be made of” refers to the material of an object, for example “this table is made of wood.”
These socks ARE made of silk is in present tense, which means you are referring to socks which still exist.
These socks WERE made of silk is in the past tense, most likely meaning the socks no longer exist (for example they were thrown away). What is interesting is the “these,” which refers to objects close to the speaker and typically refers to objects that do exist.
So we have a couple of options for the second phrase “The socks were made of silk”:
- the socks no longer exist, but the speaker is using the word “these” and not “those” to emphasize a story and make the listener feel close to the socks being described. The past tense of be “were” is used because the socks themselves existed in the past and no longer do in the present.
- the socks still exist and are close to the speaker (therefore the use of “these”), but somehow are no longer made of silk: perhaps with so many holes, they have been mended many times over with other materials and now hardly any of the original silk remains. In this case the past tense “were” refers to the “made of silk,” as the socks are no longer silk but rather the alternative material with which they have been mended.