Question:
Originally Posted by Stuffman It was the Halston Catalyst aftershave that separated and had to be shaken, to answer a previous question. Cheers for that!

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I shake 'em, I don't quite know why I do it though. Not very helpful, I know

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Hmmm... interesting topic. I never thought about shaking the bottle. My thinking is along with others that say because there does not appear to be any separation in the bottle, there is no need to do it. Also I have never noticed any real difference between the scent at the start of a bottle and that at the end of the bottle, but that could be because I cannot remember exactly what it smelled like when the bottle was new.

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I find that it takes a bit of air in some bottles before a fragrance opens up.

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Ever seen a SA shake a frag bottle before spraying you or a card? Doubt it.
Ever seen a frag box that said "Shake well before applying?" I haven't.
Ever heard an industry expert say you should shake the bottle before applying? I haven't.
Ever read on a perfume-related web site (perfumer, manufacturer, sales, blog, info, etc.) that you should shake the perfume before applying? I haven't.
Ever heard or read from a reliable sources that you SHOULDN'T shake a perfume? I haven't.
Do you know how much it gets shaken during transport from manufacturer to distributer to retailer (to eBayer) to you? A LOT!
Shake it, don't shake it. It doesn't matter. It will smell the same.
Furthermore, most perfume bottles are hermetically sealed. Air cannot get in unless you turn the bottle upside-down and spray until the tube empties of liquid. Then, if you turn the bottle right-side-up again and spray, you will see air bubbles come out of the tube on a couple of the first pumps - until the tube is full again. This is probably not a good idea. The less air in the bottle, the better. In fact, could the bottles be sealed at the factory with a slight vacuum in order to remove as much air as possible? Probably not, given that such little room remains for air in a full bottle and due to the extra costs and equipment that would be involved, but it would be possible.

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Originally Posted by MadScientist Furthermore, most perfume bottles are hermetically sealed. Air cannot get in unless you turn the bottle upside-down and spray until the tube empties of liquid. Then, if you turn the bottle right-side-up again and spray, you will see air bubbles come out of the tube on a couple of the first pumps - until the tube is full again. This is probably not a good idea. The less air in the bottle, the better. In fact, could the bottles be sealed at the factory with a slight vacuum in order to remove as much air as possible? Probably not, given that such little room remains for air in a full bottle and due to the extra costs and equipment that would be involved, but it would be possible. I'm sorry but air does get into the bottles to equalize the pressure when you spray. If it didn't you would very quickly not be able to get any fragrance out. It is simple physics. Chemistry tells you that exposure to oxygen will degrade a fragrance over time. While I agree that it will take a while, over time shaking the bottle will degrade the fragrance.

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Originally Posted by oolong I'm sorry but air does get into the bottles to equalize the pressure when you spray. If it didn't you would very quickly not be able to get any fragrance out. It is simple physics. You will not get enough of a vacuum inside a small perfume bottle to override the pump/pressure cabability of the sprayer. How is air getting into the bottle? There are no bubbles coming up from the sprayer if i turn it upside-down and spray, no matter how many times i spray. And there are no bubbles coming up from the bottom of the tube if I spray it right-side-up - everybody who has eyes can see this. So how exactly is air getting in there? If it is coming in from the top/cap upon each release of the sprayer, why are no air bubbles produced when I turn the bottle upside-down and spray? As I said, if you turn it upside down and spray until the juice in the tube is emptied, you MAY then get an exchange of air in and out of the bottle and MAY get an equalization of pressure because of the way the pump mechanism operates. It is not "simple physics," it is simple mechanics and fluid dynamics.
Ever use spray-type concentrated laundry stain removers like Spray-N-Wash? Ever notice that the sides of the bottle tend to collapse inward a bit the emptier it gets? This is because there is no air getting in. A vacuum is being created. But the bottle is small enough and the trigger-pump powerful enough that the pump mechanism can overcome the vacuum pressure. Same concept.
http://science.howstuffworks.com/question673.htm
Originally Posted by oolong Chemistry tells you that exposure to oxygen will degrade a fragrance over time. While I agree that it will take a while, over time shaking the bottle will degrade the fragrance. There is only one way oxygen might get into the bottle, as i described, and I advise not doing it because yes, over time, oxygen exposure will degrade perfume - I don't disagree with that. I am just pointing out that under normal conditions, you are not going to get additional air into the bottle AND most people (granted, BNoters don't necessarily fall into the "most people" category) will use the juice up before the air already in the bottle can do enough damage to noticeably change the scent. In other words, it's essentially a mute issue.
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Assuming the pump works as I describe and that additional oxygen does not get into the bottle under normal use, wouldn't the juice degrade only to a certain point and then stop? I have only a basic knowledge of chemistry but I would not expect that an oxygen molecule could bind to a perfume molecule, destroy it (oxidize it; change its scent), and then move on to the next perfume molecule. Doesn't the oxygen molecule bind to the perfume molecule and essentially, stay bound? If so, I conclude that the small amount of oxygen left in the bottle at production time will only alter a very tiny portion of the juice, of which is not enough to cause a perceptible change, even over a long period of time.
Light and heat are the evildoers you have to watch out for. I say you will not detect a difference in the scent of a cologne whether shaken, stirred, or calm.

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MadScientist, I like your argument, very intuitive, but I just realized one thing. How do you explain the bubbles when you shake the bottle? Bubbles are formed when a liquid surrounds an air pocket. If liquid surrounds a vacuum, the bubble collapses and you have no bubble. I don't know how the air gets in, but I assure you it does. That being said, shaking the bottle will not increase oxidation of the cologne, because oxidation will stop once you have completely used up your oxygen supply. Observe iron. If left in a sealed box, it will only oxidize do a certain point because oxidization uses up the oxygen.
- Rich
PS - Forgot to mention that shaking a bottle is not advised. Essential oils are very fragile, and even shaking them may degrade their structure.

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I understand how pump sprays work and I also understand the extreme difficulty of maintaining a vacuum over an extended time, the potentially years it takes to empty a bottle of cologne. Labs with top end equipment would be hard pressed to do that. I think the error in your logic is to assume that air infiltration occurs at the same time as the spraying rather then over time. We can argue about this all day but the next time you empty a bottle of fragrance I challenge you to force open the top. I think you will find that there is not a vacuum.

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Originally Posted by MadScientist Air cannot get in unless you turn the bottle upside-down and spray until the tube empties of liquid. Ok, I'll restate: The bottle/pump is not designed to allow air in.
Originally Posted by oolong I'm sorry but air does get into the bottles to equalize the pressure when you spray. Originally Posted by oolong I think the error in your logic is to assume that air infiltration occurs at the same time as the spraying rather then over time. I do not assume that air infiltration does not occur. YOU stated it. We're not talking precision mechanics here - just cheap spray pumps. Flaws in materials and construction will allow oxygen infiltration over time. I would not expect to hear a sucking sound if I pried off the spray pump of an empty perfume bottle that had been emptied over time. However, I wouldn't be surprised to hear a sucking sound from a bottle that one intentionally sprayed until empty and then immediately pried open.
Originally Posted by _R$_ "Bubbles are formed when a liquid surrounds an air pocket. If liquid surrounds a vacuum, the bubble collapses and you have no bubble." I think the nature of a total vacuum would preclude it from ever existing as a bubble inside a liquid, but I get your point (I'm not a physicist though so I can't intelligently get into the details of liquid/vacuum interaction). I would not expect there to be a total vacuum or even a good vacuum inside a perfume bottle, only a vacuum in the sense that the pressure is less on the inside than on the outside but not strong enough to retard the function of the spayer. There is air inside the bottle so you will get air bubbles if you shake the bottle. I haven't said anything to imply that the "pocket" of non-juice inside a perfume bottle is a total vacuum.
This really is a very interesting debate and I'm happy to continue but to sum-up and maybe clarify my original post: I have never heard, read, or seen anything to indicate that shaking perfume would affect it, bad or good. In my experience, a frag will not be perceptibly affected if shaken.