Noodler's #41 Brown is the first brown ink I wanted to buy a long time ago, but because it wasn't in stock at the time, I ended up with Polar Brown. It turns out that these two inks are so similar that they could be identical twins which only their mother can tell apart.
The main features of #41 are that it is a rich, dense brown and it is also bulletproof, which includes resistance to water and substances that forgers might use.
Because this ink and Polar Brown are almost identical in aspect, I won't go into a lot of detail but I will mention the small differences I've noticed.
I only bought a sample of #41 and I tested it in my Jinhao X750 with a broad nib.
#41 Brown comes only in Noodler's 3oz (90ml) bottles which cost $12.50 or $0.14 per milliliter. That makes it very affordable.
Color and saturation
#41 Brown is a rich, dark, as well as saturated, brown. If the nib is very thin or if the ink pools a little, you could mistake it for black. Compared to Polar Brown I believe I detect a little bit of neutrality. Polar Brown seems warmer in tone, when analyzing the q-tip color swabs.
There is very little shading and it becomes apparent only when using a thick swab or a broad nib. Because the ink is so dark don't expect the shading to show itself very often.
I didn't notice any feathering, either on Rhodia or cheap paper.
Same as with Polar Brown, #41 doesn't normally bleed through paper but if you lay it thick enough, the red component of the ink starts to show through.
Flow and lubrication
One thing that I didn't like about this ink is that it felt dry in the Jinhao. I don't want to generalize and say that many of Noodler's bulletproof inks behave similarly but this is how I feel. Often I had to coax the ink until it started to flow well, but then again, it might be the Jinhao's fault.
Drying time was very good on photocopier paper which is more absorbent, but less so on Rhodia where it took between 15 and 20 seconds for the ink to dry safely. From my experience, inks that dry quickly sometimes also bleed through the paper a little, so it's a trade-off you have to consider.
Smearing when dry
The thing that bugs me most about Noodler's Polar Brown is that, if you touch it with your finger or hand, it smears, even after it has had time to dry up. Come back a few weeks, or even months later after you've written something, rub it hard with your finger and the ink will get smudged. That isn't something I like, not when using this ink in a personal journal with 90g Clairefontaine paper.
The better quality the paper, the higher the smearing problem becomes, unfortunately.
I'm afraid that #41 Brown also smears. The good news is that it does it to a lesser extent. On regular, photocopy paper I had to rub it very hard indeed to see a little amount of smearing, while on Rhodia it performed slightly better than Polar Brown.
Smearing can be attenuated or even eliminated if you decide to dilute the ink a bit in water, at the loss of some saturation and darkness.
Since this ink is classified as "bulletproof" there are no surprises here. It is completely impervious to water. My water resistance test is done by holding the sample for a set time (>1 minute in this case) under running water. You will notice that... there isn't anything to notice. You can't even tell that the paper was submerged.
Noodler's #41 Brown is another solid ink from Noodler's, incredibly similar to Polar Brown. The differences between the two are very subtle but essentially they amount to Polar Brown being resistant to cold (useful for keeping a fountain pen in your car in winter) and #41 being slightly less prone to smearing. If you don't need cold resistance you could go for #41. Because it is dark and somber, this ink can definitely be used in a business setting. Also because it is so dark, it can be diluted to a big extent with water, greatly increasing its capacity in the process. Noodler's #41 Brown is not perfect but it is probably one of the best dark brown inks you will find.
Here are the two samples I wrote with my Jinhao X750 (broad nib) on photocopier and Rhodia paper, respectively.