Cracked Glass ?!?

[OctaviousMonk]

This one is for those with glass experience please.
I have a used Inter-American 4ft 150 gallon (three side low iron glass)
Euro-braced with two corner overflows, 1.25" drain in each box, three holes drilled on the back wall, one 1.5" bulkhead in the center and two 1" bulkheads in the upper corners. a second layer of black mirror on the back (have a crack in it but does not affect the visual appearance and integrity of the tank). The tank dimensions are 48"L x 24"T x 30"W.

Right now I have it stored in the garage while building a stand.
Yesterday I was removing the glass over flow boxes and made a horrible discovery. There seems to be what looks like a hairline verticle crack right behind where one of the glass panes met the back wall. Because of the mirrored back I can't see the other side. I also can't water test it as I need to buy bulkheads for all the holes. I can't even fathom how the hell a crack got there. It hasn't grown since I found it and looks like it could even be a deap scratch.
Does anyone have any suggestions? Perhapes siliconing a piece of glass over it? Also could the mirrored back help support the crack and keep it from spreading?

I put some masking tape on either side in the pics to highlight the crack.

 

[danny13oy]

it looks more like a scratch on the glass surface. carefully run your fingers on both sides of the crack ... do you feel sharp edges? If its only one side then its definitely a scratch ... can you tell how deep the scratch is?

 

[duke62]

if it looks like its grown its a crack which is spreading.sctatches dont spread.i wouldnt use that tank again.i think a glass place would be able to remove that panel and replace it with another 1 but not 100% sure.

 

[OctaviousMonk]

I can't get to the back of the glass because of the mirror backing on the tank. It hasn't grown or spread since discovering it, even while putting pressure with my thumbs on either side of it. While using the blade to slice the silicone I couldn't see behind it like I did when doing the side panel so I very well could have scrathed the glass.
Does anyone have any suggestions on how I could confirm if it is a scratch or crack?

 

[OctaviousMonk]

I guess I will try a leak test when I get some bulkheads

 

[duke62]

if its a hair line crack it wont leak.i cracked a 50 gallon while installing it tank was almost full and im sure it was a crack.no water leaked out.what can happen is a total breakage in which case you will be knee deep in water.may not happen today or tomorrow but it will happen.

 

[kris]

Randy, looks like a scratch to me. Either way I have a bunch of bulkheads if you want.

 

[OctaviousMonk]

duke62 I understand the possible catastrophic failure, which is why I want to be certain of what I am dealing with. I have realized that like you said a leak test won't prove anything so I came up with this. A crack by definition has to pass beyond the surface of the glass into the actually thickness, thus compromising it's structural integrity. Otherwise it is merely something on the surface correct? So I scrapped off some of the mirrored finish on the back allowing light to pass through. Next I shined a bright light through the back, down the side and top of the pane and even at and angle against the mark from the front of the pane. I wasn't able to see any penetration of the mark into the actual structure of the glass from any angle I looked. Does this make sense to you guys, or am I grasping at straws?

I am thinking I made the deep scratch when I was sliding the #11 scalpel (which I had to bend up to get the correct angle) between the glass over flow box and back pane. I wasn't able to carefully visualize my cut like I had on the side pane because of the black mirror back. When I was making the cut I could hear some loud metal on glass noises but, I assumed it was the blade against the corner edge of the overflow box as that is where the pressure of the blade seemed to be.

Once again any input is appreciated, I want to make sure I am not just deluding myself with rationalizations. Does the not seeing the crack in the glass with the light shining thing make sense?

 

[tosiek]

A deep scratch is usually as compromising as a crack in the glass, maybe not right away but in time. When glass is cut it starts with a scratch and vibrations and pressure. Im sure it happened when you cut away the silicone with the blade. Either way id be careful. Siliconing a piece of glass ontop will and will not structurally support it enough to prevent further damage to the tank in case that scratch descides on turning into a crack and messing up your room with water damage. It all depends on how close to the corner, stressses on that pane of glass, ect. We don't know the structural makup of the tank. Try to get some shots of where it is and how the tanks built as it would help alot.

Just remember, structurally the crack already weakened the glass, and the vibrations are going to stay inside that scratch for a while. Pressure once you add water might cause it to go through eventually. All depends if you can deal with a big clean up or not.

check into Elos earthquake resistant bottoms on their tanks. Its pretty much 2-3 layers of pieces of glass sandwiched together. Give you a rough idea on the glass piece patch up. but you would have to get to both sides of the tank and make sure the pane wouldn;'t fall back applying pressure to the other two patch pieces if you do go that route to fix the tank. 150g in a 20+" high tank is alot of weight on front and back panels. You would not be able to hold the front pane to the side panes if the silicone was gone.