I've got this sentence and I'm really in confusion about the right tense.
Over periods of decades and even centuries, pipes are made of various materials. or Over periods of decades and even centuries, pipes have been (being) made of various materials. In any case I'd like to emphasize that they started to make pipes of various materials a long time ago and they still do it now.
A construction like "Over periods of < timespan >" involving a period of time necessarily starts in the past. That is, it has a beginning before now and in this case the correct tense would be past tense, whether imperfect, perfect, pluperfect, etc.
Three ways of writing this, from most common to less common, are:
Over periods of decades and even centuries, pipes have been made of various materials.
(about 233,000,000 results in Google)Over periods of decades and even centuries, pipes were made of various materials.
(about 175,000,000 results in Google)Over periods of decades and even centuries, pipes were being made of various materials.
(about 13,600,000 results in Google)
For the case where you would like to emphasize that they started to make pipes of various materials a long time ago and they still do it now, you need to use a present form. You can continue from one of the previous cases if e.g. you wish to elaborate on the topic or make some distinction between the past and the present:
... of various materials, but are now most commonly made from < material >.
... of various materials, and this is still the case today.
It is possibly easier to change the structure to accommodate the present, e.g.
Pipes are made of various materials, as they have been for decades and even centuries.
But, the right tense in a sentence after "over periods of" in this case is the past tense.
