I was curious about what the probabilities were of getting the glow effect on a weapon using the fusion system. So I went ahead and figured them out.

I wasn't sure if the maximum number of times you could use fusion on a weapon was 9, 10, or 11, so these calculations are assuming you can only fuse 10 times.

Also, I did these calculations assuming that you would pick the same item to fuse with 10 times. You can of course pick different items to maximize you chances even further, but there are quite a few different combinations so I can't list them all here.

(also note these calculations are only for weapons. If you want to know about armor, multiply the max, avg columns by 2, and use 42 instead of 21, 82 instead of 41, and 102 instead of 51)

And, these calculations are assuming the 100%, 70%, 60%, etc.. chances for successful fusion. I've seen places where they state that the chances are actually 100%, 90%, 80%, etc.. and I'm not certain that this is the correct one, but I chose this because it was a lower bound and because it was based off a page on the korean site so I trust it more.

first, here is how I did my calculations so you know I'm not making these numbers up hehe:
n: number of successful fusions out of 10 you want to know about
C(i): chance that the item with +i point bonus will successfully fuse once

Number of different ways you can get n out of 10 successful fusions:
(10! / (n! * (10 - n)!)) = N(n)

For each of the N ways of getting n out of 10 successful fusions with the item that gives you +i points, the chance of getting that particular overall outcome is:
C(i)^n * (1 - C(i))^(10 - n) = M(n, i)

Probability of getting exactly n out of 10 successful fusions for an item with c chance of successful fusion:
N(n) * M(n, i) = P(n, i)

And, the probability of getting n OR MORE successful fusions out of 10 with item i is this:
P(n, i) + P(n + 1, i) + ... + P(10, i)

So, here is the table:

bonus: how many points the item is worth
%: the chance that the item will give you the point on one try
max: the maximum amount of points possible using only this item
avg: the average amount of points
req: how many successes you need to get >21, >41, and >51 points respectively
>21%: the probability of getting more than 21 points with this item
>41%: same as >21%, but for 41 points
>51%: same as >41%, but with 51 points
item bonus % max avg req >21% >41% >51%
box +1 100% +10 +10 x/x/x 0% 0% 0%
ring +2 70% +20 +14 x/x/x 0% 0% 0%
neck +3 60% +30 +18 7/x/x 38.228% 0% 0%
mirror +4 50% +40 +20 6/x/x 37.695% 0% 0%
bead +5 40% +50 +20 5/9/x 36.690% 0.1678% 0%
tally +6 30% +60 +18 4/7/9 35.039% 1.0592% 0.014369%
gourd +7 15% +70 +10.5 3/6/8 17.980% 0.1383% 0.000867%
orb +8 10% +80 +8 3/6/7 7.0191% 0.0147% 0.000912%
stone +9 5% +90 +4.5 3/5/6 1.1504% 0.0064% 0.000275%
charm +10 2% +100 +2 3/5/6 0.0864% 7.4e-5% 1.25423e-6%


Conclusion:
The 21 point glow is the only relatively easily obtainable glow effect, with the best item to use being the necklace with a 38% chance of getting it. The best you can do with the 41 point is the tally, with a 1% chance of getting it. The chances of getting a 51 point weapon are at best 1 hundredth of a percent also with the tally. So basically, unless you have a few years to spend grinding for souls, you need a cash shop item to get a red 51 point glow.

BUT, all is not lost. Generally, you can increase your overall chances of getting higher glows by starting with high point fusions, and then doing lower point fusions if the higher ones are successful, or starting over with a different item if they are not.

For example, you could use a charm on a fresh weapon, throwing out the weapon and buying a new one if the fusion fails. Once you get +10 on your first fusion, your chances after that of getting +41 and +51 are much greater, since you are now shooting for +31 and +41 in 9 fusions.

Hope this helps put things in perspective.