In what feels like decades ago in social media lifetimes, mirror glazes were quite the hype in 2016. Beautifully shiny and colorful glazed cakes were everywhere. Whether they were space or galaxy-themed, simply stylish, or brightly colored, they looked amazing and so shiny.
Even though mirror glazes were real hype for a while, they were far from new. They have been used for quite from time in French pastry. Once you know, you’ll recognize the glaze being used in a lot of delicate high-end French pastries (e.g. on those beautiful shiny dome pastries).
To me, it was pretty amazing just how shiny those mirror glazes could be. You could almost see yourself reflected in the glaze. In order to figure out why they were so shiny, we’ll have to dive deeper into the science of a mirror glaze!
What is a mirror glaze?
A mirror glaze is a way to glaze a cake that produces a super smooth, highly reflective, glossy surface. The glaze itself, once set, is very thin, maybe a millimeter thick or so. It’s like a very thin layer of jelly surrounding the cake. It doesn’t necessarily add a lot of flavor to the cake, but it does add a lot of visual appeal!
There are mirror glazes in all sorts of colors and flavors. Chocolate mirror glazes are often used, but so are fruit-flavored, e.g. strawberry, glazes. These glazes can add a little bit of flavor. However, since the layer is so thin it’s really mostly there for decoration.
How to make a mirror glaze
Before looking at why this mirror glaze is so shiny, we have to know how it’s made and what it’s made of. That will give us some clues as to why it behaves the way it does.
Short summary of the steps involved
Making a mirror glaze isn’t very complicated, except for the very last step, determining just when exactly to pour that glaze over your creation. You start a mirror glaze by dissolving gelatin (more on the role of gelatin later) in some water. You then mix the other ingredients together and heat them until all ingredients are either dissolved (e.g. sugar) or melted (e.g. cocoa fats). Finally, you mix in your pre-hydrated gelatin and leave the glaze to cool to the just-right temperature (more on that later as well). Once the mix has cooled enough, but not too much, you pour it over your creation, in one go.
Ingredients of mirror glaze
In order for a mirror glaze to work it needs to be liquid when you pour it, but it should just set when it touches the cake. One of the major ingredients to make this happen is gelatin. Just about all mirror glaze recipes will contain some amount of it and replacing gelatin isn’t easy.
Gelatin is made from collagen, which is abundant in the skins and hides of animals. We humans contain a lot of collagen as well. Collagen is then treated (hydrolyzed) to make gelatin. Gelatin is a mix of smaller proteins that dissolve well in water. They have a special interaction with water and can form a network within it, transforming something liquid into a gel.
The gel that gelatin makes is naturally glossy, thanks to the structuring of the molecules in the gel. Researchers confirmed this by using high-grade microscopy techniques (SEM) to study the surfaces of gelatin gels and those of others (Ward, 1997).
In order to activate and hydrate the gelatin, you need water. But water is also helpful for getting just the right consistency. Adding some extra water will thin the glaze, without impacting flavor.
Water can be added as such, or as part of other liquid ingredients such as water and milk. Apart from impacting consistency, the presence of water is essential to dissolve the sugars. In order to get a smooth mirror glaze, it should contain no particles at all. Having enough (warm) water just does that.
Sugar gives your glaze sweetness but also helps to slightly thicken the mixture. As mentioned before, it is very important that all the sugar is dissolved completely. To help ensure all the sugars dissolve, you will generally have to heat up the water, dissolving sugar goes faster at these higher temperatures.
Your recipe may contains just regular sugar (which is made up of the molecule sucrose) or a mix, also containing corn/glucose syrup. Glucose syrup is slightly less sweet than sucrose, but does help create that slightly thicker consistency.
Chocolate or cocoa powder
Next up: some sort of cocoa, whether it’s chocolate, or cocoa powder. A lot of recipes contain either chocolate (white, milk or dark) or cocoa powder. Chocolate is solid at room temperature so it will help solidify the glaze.
Also, the fat in chocolate, the cocoa butter, can make a glossy surface due to the way these fat crystals crystallize. Because these fats crystallize they will also make the glaze non-transparent. Without the chocolate (or cocoa butter), the glaze will remain slightly translucent.
Last but not least, the chocolate that you use will surely impact how easily the chocolate flows which is why some recipes will mention a very specific chocolate. Some chocolates flows more easily than others and so will impact how well the glaze covers your cake.
Condensed milk / cream
Dairy ingredients are also very common in a mirror glaze. You will find recipes using condensed milk or cream. Dairy contributes some richness to the glaze thanks to the proteins and some fat. As long as all the ingredients are well dissolved and don’t form clumps, it shouldn’t prevent reflectiveness. However, they’re not the main reason the glaze turns reflective.
Just like the chocolate components, the dairy ingredients can prevent a glaze from being translucent.
Flavors & Colors – optional
These two ingredients, colors & flavors don’t really have anything to do with the reflectivity of your glaze. You will probably use only very small quantities and literally all they do is add colour or flavour to your
What makes something shiny?
Before looking into why a mirror glaze is shiny it will help if we know what makes something so shiny and reflective or even why we see things.
To start with that last thing. We see things because something reflects light and that light then enters our eyes. Our brain then converts that incoming signal into an actual image. A lot of light from all around you gets reflected continuously allowing you to see. Of course, if there is no light and it’s dark outside, you can’t see anything.
Most objects reflect light in a lot of different reactions at the same time. This is called diffuse reflection. A leaf on a tree will reflect its light all around it, allowing the light to fall into your eye, but also in that of other people around you.
A mirror reflects light very consistently
Some objects, like a mirror, reflect light differently though. Mirrors are very special in that they reflect light in a very specific way, allowing you to see yourself in the mirror. This is not diffuse reflection, the light doesn’t get reflected in a lot of different directions. Instead, the mirror will reflect that incoming light beams will the exact same angle as with which they landed on the mirror. This way, the light only leaves the mirror in one direction.
By reflecting the light in a very specific angle instead of scattering it around you can imagine that an image that falls onto the mirror, will come out in the same way. All the different light beams are still organized in the same way.
Properties required to behave like a mirror
In order for a surface to behave like a mirror the first requirement is that it should be very smooth and flat. Have you ever seen the mountains or houses around a lake being reflected in it? This only happens if the lake is very smooth though. However, when there is a lot of wind and the water is rough, this will not happen anymore. The light becomes reflected in a lot of different ways so will not reflect the image of the mountains or houses anymore.
Requirement 1: A smooth surface
Since something becomes shiny due to the reflection of light on a very smooth surface, the first thing you need to make a proper mirror glaze, is a very smooth surface to pour the glaze on. If the surface is very uneven it cannot spread out properly and become reflective.
In order for the glaze to become smooth, that also means that the layer under the glaze shouldn’t absorb or otherwise interact with the glaze. If the glaze would sink into the little air bubbles in your cake, or if it would melt into the glaze, you’ve lost that smoothness as well.
Requirement 1b: No air bubbles
Since you need such a smooth surface you don’t want any air bubbles in your mirror glaze. This is why you will find a lot of recipes for mirror glazes asking you to use an immersion blender to mix the ingredients. An immersion blender is a good way to prevent air bubbles since it will stay under the surface of the glaze mixture. That way, it doesn’t pull in any air while mixing. If you’re whisking with a fork or especially whisk though you’ll be incorporating air all the time.
In most cases though your ingredients aren’t that hard to mix so you could just use a fork or spoon. Take care to be gentle so you don’t incorporate any excess air.
Requirement 2: Glaze should be thick enough
For your smooth,reflective layer to work, it should be thick enough. A very thin layer of mirror glaze may not create a smooth enough surface, with other parts sticking through.
If you pour a mirror glaze onto a cake when it is way too hot, a it will be so fluid that most of it runs off again (or worse, melts the layer underneath). That will result in an imperfect shine.
Requirement 3: Ingredient(s) that become glossy
You might be wondering, but a smooth butter frosting doesn’t become shiny even if it’s very smooth!? Correct, yes, so it’s not just the smoothness here that helps. It should also be smoother on a smaller level. The particles in your layer shouldn’t all reflect the light differently.
The way you make your glaze, as well as the ingredients in it will all impact the reflectivity. There are a lot of different recipes for mirror glazes (see below for the recipe we tested). They have a few ingredients in common so we’ll discuss their roles in mirror glaze science.
How a mirror glaze can lose its shine
A mirror glaze, once properly set and firm should be able to keep its glaze for days on end. The glaze we made stayed nice and reflective for over a week when stored in the fridge.
A good amount of gelatin in your glaze will definitely help stabilize the gloss. However, there are several ways your glaze can lose its shininess.
The first one occurs if you pour a glaze when it’s too hot. The high temperature may melt the layer underneath. As a result, the ingredients of the different layers will start to interact and your glaze might all of a sudden have particles in it which it didn’t have before. This will not be good for the mirror like properties.
Second, condensation may affect your glaze, especially when it is not yet fully set. Condensation of water on top of your glaze occurs when the cake is cold enough for moisture from the air to turn liquid on top of the cake. Whether this depends, depends on the climate in your kitchen. If it is very humid, water from the air will condense on your cake more easily. If this happens when the glaze isn’t yet fully set it may inhibit proper gelation and affect the mirror like properties.
Do you have a rubbery mirror glaze?
The gelatin in a mirror glaze continues to set for at least another day or so after making the glaze. As a result, the mirror glaze will firm up further. If you’ve used too much gelatin it can become rubbery.
Another reason for a mirror glaze turning rubbery is because of it drying out. If you don’t protect the cake from moisture loss (e.g. just store it on a plate in the fridge, what we did) it will lose moisture and dry out slowly. As a result, the glaze can become a lot more rubbery. Even though it might be a little harder to cut it will still taste good. Since the gelatin melts almost immediately in your mouth you will probably barely notice!
- A 20cm diameter cake with a smooth outside (e.g. a mousse or smooth frosting). Make sure it is at least fridge temperature, if you want you can freeze it as well.
- 7g powdered gelatin + 15g water to pre-soak*
- 85g water
- 80g cream
- 40g cocoa powder
- 120g sugar
- Pre-soak the gelatin in cold water until you need it. The powder will absorb all the moisture and become gel like, that's ok.
- Mix the other ingredients in a small sauce pan and bring to the boil. Once boiling, take it off the heat and stir through the gelatin. The gelatin will now dissolve in the mixture because it is nice and warm.
- Cool down the glaze. Most recipes advise cooling to down to 30-40C (90-110F). At this temperature it won't melt down your cake, but still be fluid enough to poor. In your case, have a look at the glaze itself. When it still pours down in a nice liquid strand, it's warm enough, use your own judgement to determine whether it will melt away your cake topping.
- Once the glaze is at the right temperature, ours was just warm to the touch, pour it over in one go and ensure it drips down nicely along the sides. It is best to do this while your cake it either on a rack or on a stand that has a smaller diameter than the cake so it will drizzle along nicely.
- Once you're poured the glaze over it will set quite fast since the cake will be cold and the layer thin, this makes the gelatin set nice and quick but it means you've got to keep working at a nice pace.
* There are a lot of different gelatins with different strengths out there. If you find your glaze comes out a little rubbery you may want to decrease the amount you need. If it doesn't set properly, you can try adding a little more. You can also use sheet gelatin, you probably need about twice as much but we didn't test it.
Chef Iso, Mirror glaze cake recipe and tutorial, link ; this website is one of the best I found to learn how to make the glaze with clear instructions and simple short videos
Barry Callebaut, Troubleshooting: glazes, link ; some good explanations on how chocolate influences the glaze’s properties
Science learning hub, Reflection of light, link ; shows some nice visualizations
Exploratorium, Laser Jello, link
Gelatin Manufacturers Institute of America, Gelatin Manual, link
G. Ward, A. Nussinovitch, Characterizing the gloss properties of hydrocolloid films, 1997, link
French chef at home, Red mirror glaze, link