Sometimes the trivet is responsible for this fine black spots. Some trivet materials produces fine scales and flicks out while cooling and settles on enamel layer. This used to happen with me and when I finely observed found this problem. Now I changed the trivet and problem is solved. 

Please do check proper particle size of flux.

regards.

Good morning Aarti,

I scanned all the great answers below and think you have all of the pieces of the puzzle laid out well.

I can't find your photo (I am not good with computers), alas. My first thought, since your first firings gave you ideal results, is that you have some contamination(typically black) of some kind introduced into your glass that was not present before...or you are being hasty with enthusiasm and your enamel is still too  damp to fire safely. Two choices. 

Contamination:

I don't see that you said what metal you are using--but if you are using any metal which creates fire-scale upon being fired and cooled, that scale can flake off and get into your work area. Copper is one of these metals which can be dirty.

Drag your dampened finger over your kiln-top, over the area where your unfired enamels rest before firing, over the area where your fired enamels rest to cool----and you will see how you stand on cleanliness on that possible source of contamination. Whatever sticks to your dampened finger could get into your work while it rests in that area.

My advice is to double check your firing area( dirty work area), keep all your firing tools only in that area (including your trivets which will become dirty with use).

Before going into your cleanest workspace( the sifting&inlay work area), pass your enamel through water and grind your exposed metal edges free of the firescale. Also be sure you clothes and hair are clean. If you have air-conditioning or a fan turned on, this will circulate air and dust particles  with the air and may,alas, get into your work. Better to not have breezes of any kind while you inlay or sift.

I once had a teacher take my class with three other students and the five of us noted many white speckles in our work that day. Never had them before. We discussed contamination that session and figured out the problem. We found out that our teacher had been working in clay with her students that day---and I believe she had some dust still upon her hair and clothing. It was a good lesson for us all, and we never had that problem again.

Those speckles were a white contamination.

If you are working on copper and your speckles are black, I think you have firescale contamination.

Wet Adhesive:

If your flaws are black or red spots, you likely have bubbles (I see someone below commented on wet adhesives) bursting during the firing due to the moisture suddenly becoming hot (boiling) and pushing aside the glass from an area on your base. As it continues firing if the glass collapses back into the exposed pit, covering the flaw, it tends to mark the area with a red color (copper oxide plus flux goes red). Black would suggest copper oxide without any glass covering it. You can often pick at the black edges with a sharp graver and feel the exposed base. It drags on your graver. That should help you decide what your problem is.

If the flaws are large--I would say it is due to moisture "boiling" and popping off the powder before it gets hot enough to adhere to the metal. Drying more fully will solve that problem entirely.

I "toast" my enamels before firing them. I lift a trivet, hold it into the kiln for the count of five, and bring it back out for a count of five & repeat and set the trivet in a clean area beside the kiln (I keep lots of wet sponges handy). Then when the kiln is back up to temperature, I do my usual firing. There is a loss of heat in the kiln box, but my enamels are always safe and dry. And clean.

Was any info here new or helpful?

Well wishing,

Diane