"Can anyone advice me how to reduce or remove the porous marks from my castings?" (Kevin)

"Porosity in castings is best dealt with by avoiding its formation in the first place...." (Andrew)

I wholeheartedly agree with Andrew's statement above, but I read Andrew's further advice and I disagree with his assertion that you can avoid pinhole porosity with shrinkage reservoirs, and also that you should necessarily cast heavier designs into lower temperature molds. It's a disservice that this is still being taught as the solution when low temperatures may well be the most common cause, along with dirty contaminated wax and loose particles of investment in the molds. This condition was studied exhaustively thirty years ago by real metallurgists working for the dental industry (which is where modern centrifugal and vacuum casting were developed), complete with microphotographs of the metallic crystal structure, graphs, charts, statistics, and more technical information than I'm willing to type, let alone pretend to understand. My information is from a dental technician's text: Skinner's SCIENCE OF DENTAL MATERIALS. If you really want to know what's happening I'd highly recommend a trip to a good library.

The reservoir method works for large shrinkage problems, especially in massive castings as in sculpture, but heavy sprues and heavy buttons are all the reserve metal needed for jewelry (in fact internal reservoirs can suck metal from your design!) and they won't solve the common condition of finding an apparently perfect casting with pinhole porosity just beneath the surface. This is usually the result of casting into a mold that has chilled below the optimum temperature, or casting metal that is very close to its solidifying temperature, or both.

What's happening is the molten metal slams into a relatively cold surface and chills instantly, causing shrinkage within the first millimeter even though there is more molten metal pushing in behind it. If the rest of the metal solidifies quickly too, the pores will remain no matter how many or how massive the reservoirs are.

The solution is to allow the metal more time to solidify by using a hotter mold (and perhaps hotter metal too), so that the initial chilling at the mold surface is extremely shallow, and the rest of the molten metal remains liquid long enough to draw in more metal from the sprues and button as it shrinks. It's best if those initial porosities never form, but even if they do they are collapsed again as the rest of the metal is hot enough to partly remelt the first chilled section.

Dental technicians have stricter conditions to fulfill, and I personally don't attempt all the exactitude they consider important. To me, casting is the art of a compromise between avoiding a slightly rough surface caused by hot molds and avoiding porosities caused by cold molds.

Perfection happens now and then... More often now that I visit the library.