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Stovetops: Aluminium vs Stainless Steel

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  • #31
    Jeez... You need to be a little more selective with your info sources.!
    ..or do you actually believe that rubbish about " pockets of air" allowing it to hold the heat longer ?
    ..and have you ever seen a cast Al cook pot crack if dropped ?
    Those comments are typical of someone with little more than a passing knowledge of modern metallurgy .
    But, hey if you are comfortable with an impression of cook pots and moka pots with sponge like texture,... That's OK.

    "Normal Aluminium"= aluminium without defects.

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    • #32
      Originally posted by blend52 View Post
      "Normal Aluminium"= aluminium without defects.
      There ain't such a thing as anything without defects. Everything on this planet, man-made or otherwise, has defects. It's all about degree.

      And... as for air pockets in cast aluminium... MOST cheaply cast aluminium suffers from hydrogen porosity to some degree and these porous air pockets are typically 0.04 to 0.5 mm in diameter in cast items of under 3kg in weight. These pockets of air in porous aluminium become even larger in size in cast items in excess of 3kgs. Numerous air pockets of up to half a millimeter in size can certainly add a thermos effect to cast aluminium and it doesn't take a rocket scientist to see that porous metal is, by necessity, more brittle and more subject to cracking.

      Sure... it is possible to cast aluminium in a way to minimize (but never completely eradicate) hydrogen porosity... but these methods are highly complex and costly and would not be applied to cheaper consumer goods... i.e. moka pots.

      Bottom line b52, just because you don't want to believe it don't mean it ain't true.

      Comment


      • #33
        Originally posted by Vinitasse View Post
        There ain't such a thing as anything without defects. Everything on this planet, man-made or otherwise, has defects. It's all about degree..
        OK Mr pedantic,.... let's refer to it as Aluminium made to a recognised industry standard.

        Originally posted by Vinitasse View Post
        And... as for air pockets in cast aluminium... MOST cheaply cast aluminium suffers from hydrogen porosity to some degree and these porous air pockets are typically 0.04 to 0.5 mm in diameter in cast items of under 3kg in weight. These pockets of air in porous aluminium become even larger in size in cast items in excess of 3kgs. Numerous air pockets of up to half a millimeter in size can certainly add a thermos effect to cast aluminium and it doesn't take a rocket scientist to see that porous metal is, by necessity, more brittle and more subject to cracking..
        Air pockets, porosity , etc is a defect.
        As I said earlier, anything can be made with defects,.....it just would not meet any recognised standard, and would be rejected.
        Thermos effect !! .??
        You do realise the reason cast Al cook pots hold their heat longer than sheet one, is because they are much thicker !?

        Originally posted by Vinitasse View Post
        Sure... it is possible to cast aluminium in a way to minimize (but never completely eradicate) hydrogen porosity... but these methods are highly complex and costly and would not be applied to cheaper consumer goods... i.e. moka pots.
        .
        No, neither complex or costly, and i doubt any casting shop would not be using simple degassing flux tablets which have been developed over many years to allow easy control of H2 porosity.
        It is a standard practice to prevent defects and costly waste.
        Im sure Bailetti would not be best pleased at your implying their products are cheaply made .

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        • #34
          Originally posted by blend52 View Post
          ....I have used a range of moka pots , Al and SS, and have never noticed a difference in flavours tween them....maybe my palette is not as finely tuned as others, or just maybe I am not that anal about fine analysis of brew characteristics...
          Dunno too many that would be so quick to admit that. In the case of the difference between alli VS s/st moka pots, the cupping characteristics are not even close to a "...fine analysis of brew characteristics...". More like so different they're glaring.

          Fact: aluminium moka pots season with coffee oils while stainless steel ones dont, resulting in different brew characteristics.

          Is it possible that some time soon you might stop trying to impress us with how much theoretical stuff you are able to memorise from web "research", and actually enlighten us with some real world practical knowledge on the subject of the topic?

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          • #35
            Originally posted by TOK View Post
            D
            Is it possible that some time soon you might stop trying to impress us with how much theoretical stuff you are able to memorise from web "research", and actually enlighten us with some real world practical knowledge on the subject of the topic?
            "Subject of the topic" ...choice of moka pot ?
            i have already stated, I cannot tell the difference, so I will leave any further debate on that to the established experts.

            and you really should not assume everyone needs the same information sources as you might use.

            Comment


            • #36
              Sigh....

              Taken from another topic but much more relevant here:

              Originally Posted by Yelta
              Your a very patient man TOK.



              Despite all this wonderful academic knowledge you seem to be able to spruke concerning the properties of aluminium, you have not been able to suggest why aluminium Stove tops "season" with coffee oils while stainless ones don't.

              I have personally seen lots of dodgy aluminium castings that are porous. On rare occasions those castings have been so porous that liquid actually seeped through, but in the great majority of cases outright leakage doesn't happen. That doesn't mean that the castings aren't porous to whatever degree, and all of these castings get through "quality control" and end up in the marketplace....simply put, the machines work and are fit for purpose.

              I didn't read this in internet research, I learned it first hand.

              Happy for you to stick your head in the sand and continue to split hairs and say that aluminium (presumably) under ideal conditions / grade unknown / on the moon / in a vacuum(?) isn't porous, without venturing an opinion as to why then aluminium moka pots "season" with coffee oils.

              Comment


              • #37
                Why do you repeatedly suggest that I should explain an effect ( seasoning ) that I have never claimed to have experienced ?
                seasoning is your proposal not mine, all i have done is correct the false assumption that moka pots are made from porous Aluminium castings.
                I you cannot explain your theory without resorting to false thinking, find another theory !
                the suggestion that any commercial aluminium cast container would be so "porous" that liquid would leak through is just so ludicrous I cannot believe you posted the comment as truth !
                If you insist it's true, I challenge you to state what the casting was, it's use and where it was made .
                any commercial cast shop would simply throw such a piece back into the recycle scrap pile.

                Comment


                • #38
                  Originally posted by blend52 View Post
                  Why do you repeatedly suggest that I should explain an effect ( seasoning ) that I have never claimed to have experienced ?
                  seasoning is your proposal not mine, all i have done is correct the false assumption that moka pots are made from porous Aluminium castings.
                  I you cannot explain your theory without resorting to false thinking, find another theory !
                  .
                  thanks Mr Popper. The obvious question is, why interject in a debate concerning the explanation for a phenomenon for which you believe no explanation is required (coz you are not convinced that the phenomenon occurs)? This is bound to descend to circularity.

                  I may as well ask 'Why are all dogs green?', ridicule the pro-photosynthesists, then after a few minutes later point out that dogs aren't green anyway.

                  Just sayin...

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                  • #39
                    Not sure which pic is most appropriate here, so I'm going to attach both
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                    • #40
                      Originally posted by Barry O'Speedwagon View Post
                      ..... why interject in a debate concerning the explanation for a phenomenon for which you believe no explanation is required (coz you are not convinced that the phenomenon occurs)?
                      Just sayin...
                      When have I said no explanation is required ?
                      where did I say I am not convinced it occurs ?
                      If you followed the discussion you would realise that my "interjection" was only to correct the blatantly false statement by Tampit....." Aluminium is porous" ....which was proposed as the explanation .

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                      • #41
                        This ongoing circular debate helps the OP how, exactly....

                        Mal.

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                        • #42
                          If the following does not settle the debate once and for all... nothing will:

                          "A PROBLEM AS OLD AS METAL ITSELF Ever since metal casting was first discovered, porosity, an area of sponge-like internal structure in an otherwise sound metal part, has been a problem. Porosity may be caused by internal shrinkage, gas cavitation, oxide films, inclusions andcombinations thereof. It can be found in virtually any type of metal casting or part, and is a particular problem in castings made from aluminum, zinc, bronze, iron, magnesium, and other alloys. Porosity is always present in powdered or sintered metal parts because of their structural nature.
                          Various methods have been used to attempt filling porous openings in parts designed to contain liquids or gases under pressure. One of the first materials used for impregnation was "water-glass" or sodium silicate. In addition to sodium silicate, tung oil, linseed oil, pitch gum and many other materials were used with little success. Shortly after World War II, the development of thermosetting plastics, to be used as impregnants, became an effective and economical means of sealing porosity within the walls of metal castings, especially when used in conjunction with vacuum pressure impregnation techniques.
                          UNDERSTANDING IMPREGNATION TECHNOLOGY
                          Impregnation in metal castings and powdered metal parts refer to the sealing of leaks resulting from porosity. The impregnating material, as a liquid, is introduced into the voids or porosity within the wall of the part usually using vacuum and pressure. The material is then solidified, filling the porous openings and making the part pressure tight.
                          Impregnation of powdered metal parts not only seals parts for pressure applications, but also improves plating or finishing, since bleedout or spotting due to entrapment of plating solutions in the pores is eliminated. Extended tool life is another benefit when machining powdered metal parts.
                          At left:
                          Coated parts
                          Part on left was impregnated prior to coating
                          Part on right was not impregnated prior to coating.
                          Note coating breakdown on part at right.

                          When castings have blind or continuous porosity areas, impregnation prior to painting or plating improves and protects the final surface finish from bleedout and blistering.
                          Impregnation technology seals leaks on all ferrous and nonferrous metals, including die castings, sand castings, investment castings, pressure castings, powdered metal parts as well as forgings or weldments. Iron, bronze, aluminum, zinc, magnesium, steel, sintered metal, as well as alloys of these metals can be impregnated. Other non-metallic materials, such as wood, plastic, and ceramics can also be impregnated.
                          IMPREGNATION IMPROVES OVERALL PART QUALITY
                          When porosity in a metal part causes leakage problems, "bad" parts are often sorted out by testing and inspection. The "good" parts that are sent to production are often as porous as the "bad" parts, but the porosity is blind and not completely interconnected. Subsequent machining, mechanical or thermal shock, or stress often breaks the thin membrane which keeps the blind porosity from being continuous, thus causing a "leaker". Impregnation fills porosity from both sides preventing leaks even if the membrane does break. Therefore, impregnation improves and enhances quality, while inspection only sorts out leakers.
                          ECONOMIES OF IMPREGNATION
                          The value added to metal parts by machining, handling, and assembly may range into the hundreds or even thousands of dollars. This value is lost when a metal part is scrapped because of porosity and leaking. Impregnation costs are small fractions of the costs of remelting, recasting, re-machining and part overruns. Impregnation allows the manufacturer to save time, money, energy and insure quality by salvaging parts which would otherwise have to be rejected. The elimination of scrap and rework substantially increases productivity. In addition, 100% impregnation of metal parts sometimes eliminates the need for expensive leak testing, and often results in a dramatic reduction of field rejects in products such as transmission cases, air-conditioners, pumps and other metal parts.
                          Impregnation of powdered metal parts provides the added benefit of prolonged tool life (up to 100 times) because IMPCO resins serve as lubricants as well as supporting the individual powered metal particles. Lubricity eliminates the chatter effect during the machining process of unimpregnated powdered metal parts.
                          Because of the proven effectiveness and economies of impregnation, many engineers specify its use for all types of metal parts that must contain liquids or gases under pressure. It is now common for impregnation processes to be incorporated directly into production schedules to insure quality, rather than to be used strictly as a salvage operation.
                          MACRO- & MICRO-POROSITY
                          There are two general classifications of porosity found in metal parts: macro-porosity in the form of large flaws in the part which may be visible to the naked eye; and micro-porosity in the form of very small, almost invisible voids. In powdered metal parts, the structure of the metal results in a condition similar to macro-porosity in castings having low density, and micro-porosity in high density castings.
                          Porosity can be found as "continuous, blind or totally enclosed" (see diagram below). Continuous porosity stretches completely through the wall thickness of a metal part causing a leakage path. Blind porosity is connected only to one side of the part wall. Totally enclosed porosity is totally isolated within the wall thickness of a part. When castings are machined, both blind and totally enclosed porosity are often "opened up" becoming continuous porosity and causing leaks.
                          Modern "Impregnation Technology" permanently seals porosity leaks caused by either micro- or macro-porosity.
                          IMPREGNATION METHODS
                          There are four common methods of impregnation consisting of dry vacuum-pressure, internal pressure, wet vacuum-pressure and wet vacuum only.
                          The dry vacuum-pressure which IMPCO pioneered is accomplished as follows:
                          1. within an autoclave a vacuum is drawn, the air in the pores is evacuated without
                          an impregnating liquid present to impede the evacuation (Figure 1) to a level fo 15 to 35 torr.
                          2. the liquid impregnant is introduced while the parts are still under vacuum (Figure 2)
                          3. a pressure cycle, up to 80-90 psi of shop air pressure (or up to six atmospheres) forces the impregnant deep into the porous cavities of the part for more positive sealing (Figure 3).
                          After the impregnation cycle the part is removed from the autoclave, the surface is then rinsed in plain water, leaving no evidence or film of the impregnating material on the part surface. Machined surfaces or tolerances are not affected. The liquid material in the pores is cured by the application of heat.
                          Internal impregnation is accomplished by placing the impregnant inside the casting and applying hydraulic pressure. This procedure is utilized in extremely large castings, forcing the liquid impregnant through the leak paths in the casting wall.
                          Wet vacuum-pressure and wet vacuum only differ in the application of pressure. They both introduce parts into an impregnant bath and evacuate the air above the bath and subsequently from the porosity of the parts through the surrounding liquid impregnant. Pressure, either atmospheric or shop air is then applied to aid in penetration of sealant.

                          After internal or wet vacuum impregnation, parts are washed and heated to solidify the resin.



                          REQUIREMENTS FOR IMPREGNANTS
                          The United States Department of Defense has established various military specifications outlining the requirements for impregnating processes and impregnants. In order to meet the standards required to produce pressure-tight castings, the ideal impregnant must be capable of penetrating and filling the porosity and then solidifying completely within the porosity of the metal parts. The impregnant should be a polar, low viscosity liquid containing no inert solvents, no filterable solid materials in suspension and producing no gaseous or liquid by-products on curing or transforming into an impervious solid.* These properties allow the impregnant to penetrate the tiniest openings and deepest recesses of porosity by capillary action. That is, such an impregnant can be drawn in by capillary forces, where it may not be possible to push it using hydraulic pressure alone. In addition, an impregnant should be stable, have a long pot life, be easy to handle and test without introducing unacceptable health and safety hazards in the work environment.
                          * NAVORD Report 6957"

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                          • #43
                            I thought the pretty pictures might help ;-P

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                            • #44
                              If this thread was/were a horse we'd shoot it.
                              BTW Vinnitasse your previous thread was longer than 600 words 😉😉😉😉

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                              • #45
                                Originally posted by mds View Post
                                If this thread was/were a horse we'd shoot it.
                                BTW Vinnitasse your previous thread was longer than 600 words 
                                Good thing I wasn't the one who wrote it then dontcha think? Just sharing the good word so feel free to read as much, or as little as you wish. And... there's always the pretty pics for those who really couldn't be bothered.

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