Should guncoating be treated as a metallic ?

EarlAlistair

Hey there polycount !

I'm currently working on a personal project involving a Mauser M712




I tried to mimic this darker tone with a metal material but whenever I did a PBR-validation with substance painters build in  "PBR-Validate"-Filter it colored my textures in red. After some testing I figued this is due to the albedo value being  simply to dark.  I did some research on the material, used to manufacture those guns and similar to other guns build in the 19th and 20th century its made from blued steel, giving it its signature dark color. From what I understand bluing is some sort of oxidation-process in which the surface of the treated metal is turned into a form of ironoxide, thus turning it into a nonmetal (dialectric).  So naturaly I thought in order to recreate this material I would have to use a metalic value of 0 instead of 1.  

My issue is the following: 

I tried around a little with said non-metalic material but wasn't realy able to achieve a result looking similar to the images above.  To me both guns still have a somewhat metal-like apperance. As far as I know you dont really wanna have a metallic value  other then 0 or 1, unless you are blending between a metal and  a nonmetal. Both dont really give me the result im looking for. So my question would be which one of the two would you decide on and how could this material be approached ?

poopipe

Strictly it's a dielectric

Blueing/acid etching fives you basically a form of rust - which is not metal 

It's a thin and imperfect covering though so on a standard metallic roughness shader you want to use a grey metallic value to simulate the effect.

This is perfectly fine and anyone who says otherwise does not understand the maths and Is regurgitating something they read on the internet

FrankPolygon

To add to what's already been said: Some protective coatings for iron and steel do have metallic properties. Deciding whether these type of PBR materials are flagged as metallic, non-metallic or something in between often depends on the type of coating used and what condition it's in.

PBR metalness values for most protective coatings are fairly straightforward since paints, plastics, varnishes, waxes, epoxies, ceramics, etc. all tend to be dielectric (non-conductive) materials. Using this general trend to make the assumption that all protective coatings are non-conductive often leads to some misconceptions about representing two very common protective coatings used to protect small steel parts.

Traditional bluing is a thin coating of black iron oxide Fe₃O₄ (magnetite) which is both magnetic and electrically conductive at room temperature. Magnetite is an iron oxide that's described as display[ing] almost metallic properties.¹ Since there's several different conversion processes for bluing steel, each with their own level of consistency and protection, there's a variety of colors and surface finishes.

[1] https://onlinelibrary.wiley.com/doi/book/10.1002/3527602097 page 115.

Parkerizing is a thin phosphate coating (Generally manganese phosphate Mn₃(PO₄)₂, zinc phosphate H₄O₁₂P₂Zn₃ or iron phosphate FePO₄.) that's electrically non-conductive but is susceptible to galvanic corrosion. The final color and luster of the phosphate coating tends to vary according to the conditions and type of solution used or the presence of contaminates. Final surface finish is usually quite dull but under certain conditions it can produce a surface finish that has a vaguely metallic sheen or fine sparkle to it.

Both coatings rely on their ability to hold oil as an additional protective layer against corrosion. Oils are generally good insulators but this property can change with different compositions or metallic contaminates. Once this protective layer of oil is stripped away the protective value of the underlying coating is reduced and there's a better chance of oxidation (rust) occurring. Without proper maintenance the iron will begin to corrode and form red iron oxide Fe₂O₃ which is largely non-conductive. Over time the oxidized surface tends to dull and turn varying shades of brown which is colloquially referred to as patina.

Simplicity [read ease of authoring relatively accurate material values] is one of several reasons why the PBR metalness workflow encourages artists to separate material values into being either metallic or non-metallic. Overall this approach works well most of the time and materials that display a mix of metallic and non-metallic properties fall into a third category called metalloids. These materials aren't that common and are simply exceptions to the basic principles of the metalness workflow.

According to the Substance PBR guide part 1:

When creating materials for PBR, it is helpful to think in terms of metal or non-metal. Ask yourself if the surface is metal or not. If it is, you will need to follow one set of guidelines. If it is not, you will need to follow another.

This can be a simplistic approach as some materials may not fall into these categories such as metalloids (a mix of metal and non-metal), but in the overall process of creating materials, distinguishing between metal and non-metal is a good approach and metalloids are an exception. To set up guidelines for materials, we must first understand what we are trying to create. With PBR, we can look at the properties of metals (conductors) and non-metals (insulators) to derive this set of guidelines as shown in Figure 12.

As noted at the beginning of this chapter, it is helpful to ask if a material is a metal or not when creating PBR materials. To be even more precise, the question should also include information about the state of the metal: whether it is painted, rusted or covered in another matter like dirt or grease. The material will be treated as dielectric if it is not raw metal. Depending on weathering, there could be some blending between metal and non-metal as weathering elements play a role in the reflective state of a metal.

https://academy.substance3d.com/courses/the-pbr-guide-part-1

Base material values are generally defined somewhere near the established values for the pure elements but final textures with dirt and wear layers rarely have pure element values spread uniformly across a given material. While there are some outliers (Certain types of parkerized finishes have a particular sheen that's difficult to reproduce without using blended values.) the case can be made that fresh bluing tends to skew more towards metallic while fresh parkerizing tends to skew more towards non-metallic.

All that said: it's also important to analyze the reference images to determine the overall surface quality, wear state and oxidation levels of these two types of coatings when choosing metalness values. As these two coatings age there's a higher likelihood of oxidation occurring and that would tend to mute the metalness values and move things more towards the non-metallic side. The quality of the surface finish and the amount of oil held by the coating can also change the perceived gloss and reflectivity of the coating.

Here's a couple of other discussions on the same topic that may be worth reading:

https://polycount.com/discussion/comment/2711887/#Comment_2711887

https://polycount.com/discussion/comment/2722463/#Comment_2722463

gnoop

I personally never   torture my brain with  such a dilemma and hardly ever do metallic channel  100% white.    Who cares if it's physically correct or not .      In our  render  100% metallic  always looks  weird and too "metal" of kind   you would never see in real life so I do just what looks right in-between  clear and cloudy sky slider.  

 

Besides too contrast metal/dielectric   areas create super ugly halos.  Killing  those halos with tweaking  metal channel contrast is only  thing I think about usually.   So nothing in my textures perfectly metal or dielectric.

  

poopipe

if you're doing that to simulate the effect of impurities/dirt then it's perfectly valid.