Black marks on tiles problem

[insajn]

Hello,
I've rented this apartment and hired some workers to put tiles on the ground and walls
After 9-12 hours I returned to the apartment and saw black marks on the tiles on ground and notice that the tiles on the walls don't have black marks.

I'm sorry if this is not the right place to ask this but can anyone tell me what are the possible causes for this black marks ? And what can I do to solve it ? And need to mention we haven't filled the voids between tiles yet.

 

[IRstuff]

can you point out these marks? I was expecting something like shoe scuff marks, but I don't see anything obvious. The tiles on the walls don't look to be the same type of tile as the floor tile. The grouting and placement is awful; how much did you pay these guys? Did you pay them at all? The workmanship is exceedingly poor, in my opinion.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm