A metal wire brush is a good tool for clean tile grout. But I found most of brushes which you can find in stores are too wide for cleaning grout line. They have some small size copper wire brushes, but they are still more wide than the grout line. There are two problems to use the wider wire brush: The 1st, most bristles brushing on tiles instead of on the grout since the grout is normally lower than tiles. You got low efficiency to clean grout. The 2nd, the edge bristles will spread out and stab your fingers in a moment of inattention.
I don't know why they do not sell a grout brush as narrow as grout line. I guess the narrow brush could not be put enough bristles on it to work strongly. Anyhow, if you can not find something in stores, doesn't matter, we have instructables.com, you can do it yourself.
Here is one way to make a grout cleaning brush. It is simple, practical and protect you fingers. I made it with a copper wire brush and a small piece metal sheet. The cost is less than $1.
Step 1: Materials and Tools
1 copper wire brush ($1.49 for 2pcs)
1 piece metal sheet
Step 2: Cut Metal Sheet and Attach on the Brush
Cut the metal sheet as per the brush to get it can be attach on the brush. What I did is bend metal sheet on the brush handle and top. I think small screws are also workable.
Step 3: Bend and Griping Brush Bristles
Trim and bend the metal sheet griping brush bristles to a line as narrow as the grout and only leave 1/4" bristles outside of the metal sheet.
This grout brush has all of bristles brushing on grout, good for heavy duty and protect your fingers from stabbing of spread wires.