The art world defines mixed media as use of different types of media in a single work of art. This could mean that different medias are used in succession or that the materials are mixed as a media before it is applied to the canvas. Your application can be calculated or extremely experimental. However, it is wise to be certain your support will accept your medium and that the different chemical properties of your chosen materials are compatible to guarantee a resilient finished work of art. Also consider the fact that when you're just experimenting, you should expect unexpected results.

One of the most common mixed mediums has been used in illustration is pen and ink with Gouache and/or Watercolor. This is where a light sketch is made and is drawn over with ink. Once the graphite is erased the painting is then colored with watercolor or gouache. In the same way sometimes the " Wax Resist"  method  is used where wax is used as a mask. After the Ink drawing is made the highlighted areas are colored with crayon or lightly painted with wax. The next step would be to apply a watercolor wash over it and where ever the lighter wax layer was applied the watercolor can not adhere. This process can also be utilized with oil pastels as well as wax based colored pencils (for finer lines) to define  texture in the subject drawing and using a watercolor wash in the background  or toned areas. 

Another method that is similar to ink and watercolor is Metal Point used with Casein , Tempera or Watercolor paint as well as a combination of them. These  are probably one of the oldest mixed media processes developed.  

Soft pastels and charcoal are often used with acrylic paints. This is usually done with the pastel or charcoal doing most of the work and the acrylic is used in areas that need super clean lines or the appearance of a wash. You don't want  heavy layers of acrylic, if you plan to go over it with chalk pastel or charcoal for touch up, because the acrylic will resist them. At this phase it would be better to use oil pastels or have the forethought to replace the acrylic with watercolor to get the job done.

Oil Pastels are not considered mixed media when combined with oil paints because of their chemical similarities. For this reason oil pastels are categorized as oil paint. However, I do enjoy using turpentine with a (dry brush) to create cleaner lines and textures leaving the pencil strokes intact to for consistency.

Gilding, the art of applying  gold or silver leaf to a surface via oil size (oil gilding) or rabbit skin glue (water gilding) was an art known and practiced by the ancients. I believe there is a strong possibility Casein glue was used in this process as well .  Gilding in combination with wax, oils or temperas had been a global practice during ancient times for everything from architectural decorations to religious icons. These pieces had gold leaf glued to wood gesso panels (with rabbit skin glue) and brushed out  with a gilder's tip, then trimmed and burnished. Painting generally surrounded these areas to complete the image. Gold leaf areas in paintings included intricate borders, halos, thrones, crowns and other various metal objects. Your possibilities are endless as to how you can apply this craft to enhance you work.   

As another mixed medium, encaustics have been used with oils since medieval times. Mostly wax and oil were blended with as a medium that could be brushed on  to toughen the finish at that time. However, you can achieve a wonderful variety of textures by varying the ratios of oil, wax & resins and provided the build up is not excessive it can all be applied to a well sized/grounded board.

Other than the fact that oil and water don't mix,  there is a distinct difference  between acrylics and oils on canvas. Some emulsions are nothing more than a suspension meaning that it will be difficult for it to cure or dry as a uniform application. Usually water ascends to the surface to evaporate as an emulsion dries. Oil paint dries slowly and the oil absorbs oxygen as it polymerizes. Decisively, the reason oil and watercolors/acrylics are not mixed. However, one supporting way in which oil and acrylics are used together is by applying oil paints over a dry acrylic painting. Emphasis on "dry" Acrylic which takes months.

Acrylic Latex can never go over oil with a "lasting" bond except when the oil base is a primer. Oil can paint over latex or watercolor provided it has been given adequate drying time and this time saving method is frequently used as an under-painting. If Acrylic is not dry when it is painted over with Oil, the under surface will be sealed off preventing moisture from evaporating. It would be only a matter of time until the surface cracks.

Intended only for rigid supports, adding sand, marble, other solid materials and/or slacked lime to acrylics will help to achieve special textured and relief effects. 0ther than lime these materials can be blended with your ground or used  in your painting process when applied with great flexibility. Lime can be blended with acrylic colors to achieve deep textures as well as a faux fresco appearance. 

In an attempt to recreate the mood of a summer morning after a rain, the painting   " Grazin'" of the property above was painted with the house and distant landscape painted with smooth cool toned traditional acrylic washes on  masonite panel. The foreground was painted with lime plaster mixed with warm toned acrylic colors and applied with a palette knife to stimulate the rural woodsy appearance. Although I had received several complements and recognize that artists are always more critical of their own work, I still have  mixed opinions about this piece. I am pleased since I was not obligated to place any mechanical objects in this piece that it has a slight timeless appeal . The most negative aspect is that excessive emphasis was placed on controlling perspective limiting the natural flow on this painting. In any advent I guess practice makes perfect.

Links

Since mixing mediums is a fairly modern practice, as well as a hot on campus topic that's usually considered experimental , I haven't been able to find much online instructional literature on mixed media. If anyone would care to contribute a URL for additional information you can email  me and I will gladly place it here. In the mean time I will be writing more on this subject later on, after I've completed previous commitments.

 Lesson Plan: Photo Transfer - Social Comment -Mixed Media

 Leo Monahan / Paper in Dimensions -  An excellent mixed media representation of three dimensional paintings using paper, glue and paint.