Don't know what the OG was but need it?

Hi all - I'm new around here, so forgive me if I'm repeating things you already know, I looked a bit and couldn't find it.
Anyways, there are times when you need your Original Specific Gravity reading, but perhaps you were drinking and brewing and forgot to take it, or perhaps you wrote it down on a matchcover and lost it, whatever.
All is not Lost!
Use a glass test jar or measuring cup and select a testable amount. You know, enough to float your hydrometer.
THIS AMOUNT WILL NOT BE COMPLETELY RECOVERABLE! Be stingy as you can.
Fill it to the necessary level, mark the fill level well. Very very well.
Measure with your hydrometer. This your current reading.
Now pour it into a pan, or if you feel like getting fancy, do a water bath.
You need to raise the temperature of the liquid to about 174-178F and keep it for about 10 minutes. Don't worry if it boils, as it might, depending on what stage of the ferment you are at, and what the alcohol level is. If it does, you are only losing steam in addition to the ethanol evaporating, and we can take care of that in a minute ....Now let it cool.
Don't be stupid like me and blow up your nice test glass trying to crash cool it
Once it's cool, put it into your same test vessel. If done properly, it should not come back up all the way to the original fill level.
Now, fill it to EXACTLY the original fill level with distilled water, or whatever you have.
Measure. That's your OG.
:rockin:
I hope I explained that properly, and I hope it helps.
Cheers!

I'm not sure that I follow, but the way I'm reading it won't give you your OG. Assuming that this is fermented beer you're talking about testing, you're not measuring the sugars already fermented by the yeast. For instance, let's say we have a batch with an OG of 1.060, and an FG of 1.010. If I take a sample of the 1.010 beer (which contains EtOH), boil off the EtOH, and add back water to replace it, I'll wind up higher than 1.010, but won't get close to 1.060. While the EtOH does lower the gravity, and getting rid of the EtOH will allow you to measure the gravity of just the sugars in solution (assuming you're able to get rid of all the EtOH), you won't be able to measure the sugars that were previously in solution at the OG.

Basically, the OG consists of mostly sugar (some protein, etc., but that should remain fairly constant). The FG consists of leftover sugar as well as EtOH generated from the fermentation of the sugar in the OG. Unless I'm reading your method wrong, it will give you a good calculation of the leftover sugar in the FG, but not of the amount of sugar that has already been fermented that would have contributed to the OG. I suppose that if you were able to extract just the EtOH from the sample (without any steam) you could calculate the Alc% and with the FG calculate back to the OG, but that's generally beyond what we're capable of doing.

Again, its quite possible that I'm reading this wrong, or I've missed something, so I just wanted to double check before anyone tried it and got really confused.

It's just a ballpark. If you want to get technical, here:
.
. Measure the SG of the finished wine. This will be SG1. 2. Measure out exactly one pint (or 500 ml) of the wine. This is the "Sample." 3. In an enameled or glass pan, boil the sample until it is exactly 1/2 its original volume, i.e. 1/2 pint (or 250 ml). What had been reduced in this process is some of the water and ALL of the alcohol, because alcohol boils at a lower temperature than water, about 173 degress F (78 degrees C) for alcohol vs. 212 degrees F (100 degrees C) for water. 4. Using distilled water make the sample back to the original volume, i.e. 1 pint (or 500 ml). 5. Cool the sample to the temperatrue for which your hydrometer is calibrated and measure the SG of the sample now. You should get a higher reading (now that all the alcohol is gone). This will be SG2. 6. Find the difference between the SG2 and SG1, which we will call the "spirit indication" and read the ABV from the following where the first number is the difference in SG (spirit indication) and the second is ABV:1.5=1%, 2=1.3%, 3=2%, 4=2.7%, 5=3.4%, 6=4.1%, 7=4.9%, 8=5.6%, 9=6.4%, 10=7.2%, 11=8.0%, 12=8.8%, 13=9.7%, 14=10.5%, 15=11.4%, 16=12.3%, 17=13.2%, 18=14.1%, 19= 15.1%, 20=16%, 21=17%., 22=18%, 23=19%, 24=20%, 25=21%, 26=22%__________________
Props to saramc for the reference.

Ah, so you're using it to calculate the ABV, not the OG. That makes more sense. Where its confusing is at the end of your original post when you say that the number you get is your OG, where it sounds like its actually the SG2 (from the protocol you listed). That should give you the FG from the remaining sugars, which would allow you to convert that to the ABV using the numbers you've listed. And I suppose once you've got the ABV, you should be able to use it and the FG to calculate back to the OG if you wanted to. If I've done the math correctly, this formula should get you the rest of the way there:

OG=(ABV/0.131)+FG if using gravity points,
OG=(ABV/131)+FG if using the 1.0xx for gravity

Does that work out correctly?

I'm on my phone at work but it looks right... really it's just measuring alcohol levels, but you can use it at any point in the ferment, so it's pretty handy. It's obviously not precise with sugar density and whatnot, but you can work that all out too if you're motivated... Usually if I'm doing this my airlock activity is stopped earlier than expected, and I'm just trying to figure out if it's stalled or just finished fast.
Cheers!