Instruction: Answer the given question by extracting information from the supporting document.

Input:
Question: Just used the grinder for the first time. The batter scraped off from the granite stones was black, what is it? How to avoid getting that black stuff?
Document: Initial impression: Quite larger, larger than any of my kitchen appliances. Steel is heavy and the stones don't look granite, but just stones. The stone assembly is hard to clean initially due to several nooks and crannies.Batter time: Batter was extremely smooth, too smooth to make good idlis but could have been good for dosa.Problem: The batter that I removed from the flat ends of the stones was blackish-grey.
First of all this product is priced very expensive. It got some glitches too like every time when we tried to grind rice its making some strange noise. Either we have to remove the upper handle or switch off the grinder to fix this issue.
Initially I thought this one is bit expensive for my budget but I ordered it with half the mind since I needed it desperately. Once the product arrived, now I feel it's really worth the money and time I spend for preparing batter. Highest quality wet grinder. No second thought in it.
Surey a great wet grinder. Packaging and Delivery was perfect as advertised.I've used it for more than 10 times and the performance is impressive.I'm sure if you use and maintain it properly, it will last for several years.Really satisfied.
I ordered this as a gift as I already own one. It came pretty quickly after the order and the bonus was the atta kneader was included unlike mine. I am not sure how much use the atta kneader will get as the base of the grinder is stone and rough.The person I gifted it to used it and she said it worked well. As far as this brand, I have ad it for a couple of years now.
Tried to remove as much of the grey batter as possible but some of it still got mixed into the rest of the batter. Anyhow, I couldn't give this set of idlis to my young one, although we ate and are fine so far. God knows what that substance was, I am wary to use it again. Gonna try this one more time and if I see the grey thing again, back it will go.Am surprised nobody else had this problem? !
My unit was shipped by Great Elect. I was worried that they will send me a damaged or used unit. The Item was brand new, works well and I have no complaints.
I was hesitant in buying this kind of a large specialty appliance but had been thinking about it for months. I was happy to say that shipment was timely and fast. It was packaged in its original box and then in another 2 more boxes! It came in perfect condition. For years I have used a standard blender to make idli/dosai, vada etc...but this had made a huge difference in the texture of idlis especially.
Works nice and quiet. Nicely packed and delivered on time. Very very easy to assemble and it took just 30 second to put things together and start running.
I could not yet rate this product yet . I only  used for couple of times. this product should be used many times before one cane rate this working condition and durability. This item is not used in u.s. every day. it is used  in india  every day and they are in good condition for years . I will give my rating after using at least 20 times 0r more thanks. they should give more than one year warranty
Answer: The first time, just grind a cup of rice and discard it. Wash the grinder with soap and water well after that and use for your regular use. It should be fine then. If it still persists, write to the vendor.